Section Three: From Moral Metaphysics to the Critique of Pure Practical Reason

The will is a kind of causality belonging to living beings in so far as they are rational, and freedom would be this property of such causality that it can be efficient, independently of foreign causes determining it; just as physical necessity is the property that the causality of all irrational beings has of being determined to activity by the influence of foreign causes.

The will is a kind of causality that living beings possess insofar as they are rational. Freedom would be the ability of this causality to act without being determined by outside causes. By comparison, physical necessity is a property of the causality of all nonrational beings. Their causality is determined to act by the influence of outside causes.

The preceding definition of freedom is negative and therefore unfruitful for the discovery of its essence, but it leads to a positive conception which is so much the more full and fruitful.

The preceding definition of freedom is negative, so it does not help us discover freedom’s essential nature. But it leads to a positive understanding that is all the richer and more fruitful.

Since the conception of causality involves that of laws, according to which, by something that we call cause, something else, namely the effect, must be produced; hence, although freedom is not a property of the will depending on physical laws, yet it is not for that reason lawless; on the contrary it must be a causality acting according to immutable laws, but of a peculiar kind; otherwise a free will would be an absurdity. Physical necessity is a heteronomy of the efficient causes, for every effect is possible only according to this law, that something else determines the efficient cause to exert its causality. What else then can freedom of the will be but autonomy, that is, the property of the will to be a law to itself? But the proposition: "The will is in every action a law to itself," only expresses the principle: "To act on no other maxim than that which can also have as an object itself as a universal law." Now this is precisely the formula of the categorical imperative and is the principle of morality, so that a free will and a will subject to moral laws are one and the same.

The idea of causality includes the idea of laws. According to these laws, something we call a cause must produce something else, namely an effect. So although freedom is not a property of the will governed by physical laws, freedom is not therefore lawless. On the contrary, freedom must be a kind of causality that follows unchanging laws of a special kind. Otherwise, a free will would be absurd. Physical necessity means that efficient causes are governed by something other than themselves. For every effect is possible only under this law: something else determines the efficient cause to exercise its causal power. What, then, can freedom of the will be but autonomy, the will's ability to give itself its own law? But the statement, "The will gives itself its own law in every action," expresses only this principle: "Act only on a maxim that can also have itself as a universal law as its aim." This is precisely the formula of the categorical imperative and the principle of morality. So a free will and a will subject to moral laws are one and the same.

On the hypothesis, then, of freedom of the will, morality together with its principle follows from it by mere analysis of the conception. However, the latter is a synthetic proposition; viz., an absolutely good will is that whose maxim can always include itself regarded as a universal law; for this property of its maxim can never be discovered by analysing the conception of an absolutely good will. Now such synthetic propositions are only possible in this way: that the two cognitions are connected together by their union with a third in which they are both to be found. The positive concept of freedom furnishes this third cognition, which cannot, as with physical causes, be the nature of the sensible world (in the concept of which we find conjoined the concept of something in relation as cause to something else as effect). We cannot now at once show what this third is to which freedom points us and of which we have an idea a priori, nor can we make intelligible how the concept of freedom is shown to be legitimate from principles of pure practical reason and with it the possibility of a categorical imperative; but some further preparation is required.

If we assume freedom of the will, then morality and its principle follow simply by analyzing the concept of freedom. Yet the moral principle is a synthetic proposition: an absolutely good will has a maxim that can always serve as a universal law. For this property of its maxim cannot be discovered by analyzing the concept of an absolutely good will. Such synthetic propositions are possible only when two items of knowledge are connected through a third that contains them both. The positive concept of freedom provides this third item of knowledge. Unlike the link between physical causes and effects, this third item cannot be the nature of the world of sense. In our concept of that world, we connect the concept of something as a cause with something else as its effect. We cannot yet show what this third thing is, toward which freedom points us and of which we have an a priori idea. Nor can we yet explain how principles of pure practical reason establish the legitimacy of the concept of freedom. Along with that, we cannot yet explain the possibility of a categorical imperative. Some further preparation is needed.

It is not enough to predicate freedom of our own will, from Whatever reason, if we have not sufficient grounds for predicating the same of all rational beings. For as morality serves as a law for us only because we are rational beings, it must also hold for all rational beings; and as it must be deduced simply from the property of freedom, it must be shown that freedom also is a property of all rational beings. It is not enough, then, to prove it from certain supposed experiences of human nature (which indeed is quite impossible, and it can only be shown a priori), but we must show that it belongs to the activity of all rational beings endowed with a will. Now I say every being that cannot act except under the idea of freedom is just for that reason in a practical point of view really free, that is to say, all laws which are inseparably connected with freedom have the same force for him as if his will had been shown to be free in itself by a proof theoretically conclusive.¹⁷ Now I affirm that we must attribute to every rational being which has a will that it has also the idea of freedom and acts entirely under this idea. For in such a being we conceive a reason that is practical, that is, has causality in reference to its objects. Now we cannot possibly conceive a reason consciously receiving a bias from any other quarter with respect to its judgements, for then the subject would ascribe the determination of its judgement not to its own reason, but to an impulse. It must regard itself as the author of its principles independent of foreign influences. Consequently as practical reason or as the will of a rational being it must regard itself as free, that is to say, the will of such a being cannot be a will of its own except under the idea of freedom. This idea must therefore in a practical point of view be ascribed to every rational being.

It is not enough to attribute freedom to our own will, whatever our reason for doing so. We must also have sufficient grounds to attribute freedom to all rational beings. For morality is a law for us only because we are rational beings. So morality must also apply to all rational beings. And since morality must be derived from freedom alone, we must show that all rational beings possess freedom. It is therefore not enough to prove freedom from supposed experiences of human nature. Indeed, such a proof is quite impossible. Freedom can only be established a priori, independently of experience. Instead, we must show that freedom belongs to the activity of all rational beings with a will. Now I say that any being that can act only under the idea of freedom is, for that very reason, practically free. In other words, all laws inseparably connected with freedom have the same force for that being. They apply just as if a theoretically conclusive proof had shown its will to be free in itself.¹⁷ Now I maintain that we must attribute the idea of freedom to every rational being with a will. We must also regard every such being as acting entirely under this idea. For we understand such a being to possess practical reason, meaning reason that can cause its objects to become real. Now we cannot conceive of reason as consciously letting an outside influence guide its judgments. For then the being would attribute its judgment to an impulse, not to its own reason. Reason must regard itself as the source of its principles, independent of outside influences. Consequently, as practical reason, or as the will of a rational being, reason must regard itself as free. In other words, a rational being's will can be its own only under the idea of freedom. We must therefore attribute this idea of freedom to every rational being from a practical standpoint.

We have finally reduced the definite conception of morality to the idea of freedom. This latter, however, we could not prove to be actually a property of ourselves or of human nature; only we saw that it must be presupposed if we would conceive a being as rational and conscious of its causality in respect of its actions, i.e., as endowed with a will; and so we find that on just the same grounds we must ascribe to every being endowed with reason and will this attribute of determining itself to action under the idea of its freedom.

We have finally traced the precise concept of morality back to the idea of freedom. However, we could not prove that freedom actually belongs to us or to human nature. We saw only that we must assume freedom if we are to conceive of a rational being with a will. Such a being is conscious of causing its own actions. So, on exactly the same grounds, we must attribute this capacity to every being with reason and a will: the capacity to determine its own actions under the idea that it is free.

Now it resulted also from the presupposition of these ideas that we became aware of a law that the subjective principles of action, i.e., maxims, must always be so assumed that they can also hold as objective, that is, universal principles, and so serve as universal laws of our own dictation. But why then should I subject myself to this principle and that simply as a rational being, thus also subjecting to it all other being endowed with reason? I will allow that no interest urges me to this, for that would not give a categorical imperative, but I must take an interest in it and discern how this comes to pass; for this properly an "I ought" is properly an "I would," valid for every rational being, provided only that reason determined his actions without any hindrance. But for beings that are in addition affected as we are by springs of a different kind, namely, sensibility, and in whose case that is not always done which reason alone would do, for these that necessity is expressed only as an "ought," and the subjective necessity is different from the objective.

By presupposing these ideas, we also became aware of a law governing our subjective principles of action, or maxims. We must always adopt maxims that can also hold as objective principles, valid for everyone. They can then serve as universal laws that we give ourselves. But why should I submit to this principle simply because I am a rational being? And why should I therefore subject all other rational beings to the same principle? I admit that no interest drives me to submit, since that would not produce a categorical imperative. Yet I must take an interest in this principle and understand how that interest arises. For an "I ought" is really an "I would" for every rational being, provided reason alone determined their actions without hindrance. But beings like us are also influenced by other motives, namely those arising from our senses. We do not always do what reason alone would lead us to do. For such beings, then, this necessity is expressed only as an "ought," and subjective necessity differs from objective necessity.

It seems then as if the moral law, that is, the principle of autonomy of the will, were properly speaking only presupposed in the idea of freedom, and as if we could not prove its reality and objective necessity independently. In that case we should still have gained something considerable by at least determining the true principle more exactly than had previously been done; but as regards its validity and the practical necessity of subjecting oneself to it, we should not have advanced a step. For if we were asked why the universal validity of our maxim as a law must be the condition restricting our actions, and on what we ground the worth which we assign to this manner of acting- a worth so great that there cannot be any higher interest; and if we were asked further how it happens that it is by this alone a man believes he feels his own personal worth, in comparison with which that of an agreeable or disagreeable condition is to be regarded as nothing, to these questions we could give no satisfactory answer.

It seems, then, as if the moral law is merely assumed in the idea of freedom. The moral law is the principle of autonomy of the will. It also seems as if we cannot independently prove the moral law’s reality and objective necessity. In that case, we would still have gained something important. We would at least have defined the true principle more precisely than anyone had before. But we would have made no progress in establishing its validity or the practical necessity of obeying it. For suppose someone asked why our actions must be limited to maxims that can hold universally as laws. Suppose they also asked what grounds the value we place on acting this way. We consider this value so great that no other interest can rank higher. And suppose they further asked why only by acting this way does a person believe he feels his own worth. Compared with that personal worth, the value of a pleasant or unpleasant condition must count as nothing. We could give no satisfactory answer to these questions.

We find indeed sometimes that we can take an interest in a personal quality which does not involve any interest of external condition, provided this quality makes us capable of participating in the condition in case reason were to effect the allotment; that is to say, the mere being worthy of happiness can interest of itself even without the motive of participating in this happiness. This judgement, however, is in fact only the effect of the importance of the moral law which we before presupposed (when by the idea of freedom we detach ourselves from every empirical interest); but that we ought to detach ourselves from these interests, i.e., to consider ourselves as free in action and yet as subject to certain laws, so as to find a worth simply in our own person which can compensate us for the loss of everything that gives worth to our condition; this we are not yet able to discern in this way, nor do we see how it is possible so to act- in other words, whence the moral law derives its obligation.

Sometimes we do take an interest in a personal quality without any interest in our external circumstances. But the quality must make us fit to share in those circumstances if reason were to distribute them. In other words, simply being worthy of happiness can interest us, even without the prospect of enjoying that happiness. Yet this judgment results only from the importance we have already assumed the moral law to have. We made that assumption when, through the idea of freedom, we separated ourselves from every interest based on experience. But this approach does not yet show why we ought to separate ourselves from those interests. That is, we do not yet see why we should regard ourselves as free in acting, yet subject to certain laws. In doing so, we would find a worth in ourselves alone that could compensate for losing everything valuable about our circumstances. Nor do we yet see how acting in this way is possible. In other words, we do not yet see where the moral law gets its binding force.

It must be freely admitted that there is a sort of circle here from which it seems impossible to escape. In the order of efficient causes we assume ourselves free, in order that in the order of ends we may conceive ourselves as subject to moral laws: and we afterwards conceive ourselves as subject to these laws, because we have attributed to ourselves freedom of will: for freedom and self-legislation of will are both autonomy and, therefore, are reciprocal conceptions, and for this very reason one must not be used to explain the other or give the reason of it, but at most only logical purposes to reduce apparently different notions of the same object to one single concept (as we reduce different fractions of the same value to the lowest terms).

We must freely admit that there is a kind of circular reasoning here that seems impossible to escape. In the order of efficient causes, we assume we are free. We do so to see ourselves as subject to moral laws in the order of ends. We then see ourselves as subject to these laws because we have attributed freedom of will to ourselves. For freedom and the will’s power to make its own laws are both autonomy. They are therefore concepts that imply each other. For that very reason, neither concept can explain the other or provide a basis for it. At most, we can use one concept for a logical purpose: to combine apparently different ideas about the same object into a single concept (just as we reduce different fractions of equal value to their lowest terms).

One resource remains to us, namely, to inquire whether we do not occupy different points of view when by means of freedom we think ourselves as causes efficient a priori, and when we form our conception of ourselves from our actions as effects which we see before our eyes.

One option remains: to ask whether we take different viewpoints in two cases. In the first, through freedom, we think of ourselves as causes that produce effects a priori. In the second, we form our idea of ourselves from our actions, seen as effects we can observe.

It is a remark which needs no subtle reflection to make, but which we may assume that even the commonest understanding can make, although it be after its fashion by an obscure discernment of judgement which it calls feeling, that all the "ideas" that come to us involuntarily (as those of the senses) do not enable us to know objects otherwise than as they affect us; so that what they may be in themselves remains unknown to us, and consequently that as regards "ideas" of this kind even with the closest attention and clearness that the understanding can apply to them, we can by them only attain to the knowledge of appearances, never to that of things in themselves. As soon as this distinction has once been made (perhaps merely in consequence of the difference observed between the ideas given us from without, and in which we are passive, and those that we produce simply from ourselves, and in which we show our own activity), then it follows of itself that we must admit and assume behind the appearance something else that is not an appearance, namely, the things in themselves; although we must admit that as they can never be known to us except as they affect us, we can come no nearer to them, nor can we ever know what they are in themselves. This must furnish a distinction, however crude, between a world of sense and the world of understanding, of which the former may be different according to the difference of the sensuous impressions in various observers, while the second which is its basis always remains the same, Even as to himself, a man cannot pretend to know what he is in himself from the knowledge he has by internal sensation. For as he does not as it were create himself, and does not come by the conception of himself a priori but empirically, it naturally follows that he can obtain his knowledge even of himself only by the inner sense and, consequently, only through the appearances of his nature and the way in which his consciousness is affected. At the same time beyond these characteristics of his own subject, made up of mere appearances, he must necessarily suppose something else as their basis, namely, his ego, whatever its characteristics in itself may be. Thus in respect to mere perception and receptivity of sensations he must reckon himself as belonging to the world of sense; but in respect of whatever there may be of pure activity in him (that which reaches consciousness immediately and not through affecting the senses), he must reckon himself as belonging to the intellectual world, of which, however, he has no further knowledge. To such a conclusion the reflecting man must come with respect to all the things which can be presented to him: it is probably to be met with even in persons of the commonest understanding, who, as is well known, are very much inclined to suppose behind the objects of the senses something else invisible and acting of itself. They spoil it, however, by presently sensualizing this invisible again; that is to say, wanting to make it an object of intuition, so that they do not become a whit the wiser.

The following observation requires no subtle reflection. We may assume that even the most ordinary understanding can make it. Such an understanding may reach it only through a vague judgment that it calls feeling. All the "ideas" that come to us involuntarily, such as sensory ideas, reveal objects only as those objects affect us. What the objects may be in themselves remains unknown to us. Consequently, even the closest attention and clearest understanding of "ideas" of this kind can give us only knowledge of appearances. They can never give us knowledge of things in themselves. Once we make this distinction, we must accept something behind appearances that is not itself an appearance: things in themselves. Perhaps we make the distinction simply by noticing the difference between two kinds of ideas. Some ideas come from outside us, and we receive them passively. Others we produce entirely ourselves, showing our own activity. Yet we must admit that we can know things in themselves only through how they affect us. We can therefore get no closer to them or ever know what they are in themselves. This gives us a distinction, however rough, between a world of sense and the world of understanding. The world of sense may differ because different observers receive different sensory impressions. But the world of understanding, which underlies the world of sense, always remains the same. Even knowledge gained through inner sensation cannot justify a person's claim to know what they are in themselves. For a person does not, so to speak, create themselves. Nor do they gain their conception of themselves a priori, independently of experience, but instead through experience. It naturally follows that even self-knowledge comes only through inner sense. Consequently, people know themselves only through the appearances of their nature and the ways their consciousness is affected. At the same time, they must assume something underlying these features of themselves, which consist only of appearances. This underlying something is their ego, whatever its characteristics in itself may be. Thus, as beings who merely perceive and receive sensations, people must regard themselves as belonging to the world of sense. But insofar as they possess any pure activity, they must regard themselves as belonging to the intellectual world. Pure activity reaches consciousness directly, rather than by affecting the senses. Yet they have no further knowledge of this intellectual world. Anyone who reflects must reach this conclusion about all things that can be presented to them. Probably even people of the most ordinary understanding reach this conclusion. As is well known, they readily suppose something invisible and independently active behind the objects of the senses. But they spoil this insight by then treating this invisible something as sensory again. In other words, they want to make it an object of direct perception, leaving themselves no wiser at all.

Now man really finds in himself a faculty by which he distinguishes himself from everything else, even from himself as affected by objects, and that is reason. This being pure spontaneity is even elevated above the understanding. For although the latter is a spontaneity and does not, like sense, merely contain intuitions that arise when we are affected by things (and are therefore passive), yet it cannot produce from its activity any other conceptions than those which merely serve to bring the intuitions of sense under rules and, thereby, to unite them in one consciousness, and without this use of the sensibility it could not think at all; whereas, on the contrary, reason shows so pure a spontaneity in the case of what I call ideas [ideal conceptions] that it thereby far transcends everything that the sensibility can give it, and exhibits its most important function in distinguishing the world of sense from that of understanding, and thereby prescribing the limits of the understanding itself.

Human beings do find within themselves a capacity that distinguishes them from everything else: reason. Reason distinguishes them even from themselves as affected by objects. As a purely spontaneous activity, reason stands even above the understanding. The understanding is also spontaneous, unlike the senses. The senses merely receive intuitions when things affect us, and are therefore passive. Yet the understanding’s activity can produce only concepts that bring sensory intuitions under rules, uniting them in one consciousness. Without using sensory experience in this way, the understanding could not think at all. Reason, by contrast, displays such pure spontaneity in what I call ideas [ideal conceptions] that it goes far beyond anything the senses can provide. Reason performs its most important function by distinguishing the world of sense from the world of understanding. By drawing this distinction, reason also sets the limits of the understanding itself.

For this reason a rational being must regard himself qua intelligence (not from the side of his lower faculties) as belonging not to the world of sense, but to that of understanding; hence he has two points of view from which he can regard himself, and recognise laws of the exercise of his faculties, and consequently of all his actions: first, so far as he belongs to the world of sense, he finds himself subject to laws of nature (heteronomy); secondly, as belonging to the intelligible world, under laws which being independent of nature have their foundation not in experience but in reason alone.

For this reason, a rational being must view himself as an intelligence, not in terms of his lower faculties. As an intelligence, he belongs not to the world of sense but to the world of understanding. He therefore has two points of view from which to understand himself. From each viewpoint, he can recognize laws governing the use of his faculties and, consequently, all his actions. First, insofar as he belongs to the world of sense, he is subject to the laws of nature (heteronomy). Second, insofar as he belongs to the intelligible world, he is subject to laws independent of nature. These laws are grounded not in experience but in reason alone.

As a rational being, and consequently belonging to the intelligible world, man can never conceive the causality of his own will otherwise than on condition of the idea of freedom, for independence of the determinate causes of the sensible world (an independence which reason must always ascribe to itself) is freedom. Now the idea of freedom is inseparably connected with the conception of autonomy, and this again with the universal principle of morality which is ideally the foundation of all actions of rational beings, just as the law of nature is of all phenomena.

As a rational being, a person belongs to the intelligible world, the world understood through reason. Such a person can understand their will as a cause only by assuming the idea of freedom. For freedom means independence from the determining causes of the world experienced through the senses. Reason must always regard itself as independent of those causes. Now the idea of freedom is inseparably linked to the idea of autonomy. Autonomy, in turn, is inseparably linked to the universal principle of morality. In principle, this moral principle is the foundation of all actions by rational beings, just as the law of nature is the foundation of all observable phenomena.

Now the suspicion is removed which we raised above, that there was a latent circle involved in our reasoning from freedom to autonomy, and from this to the moral law, viz.: that we laid down the idea of freedom because of the moral law only that we might afterwards in turn infer the latter from freedom, and that consequently we could assign no reason at all for this law, but could only [present] it as a petitio principii which well disposed minds would gladly concede to us, but which we could never put forward as a provable proposition. For now we see that, when we conceive ourselves as free, we transfer ourselves into the world of understanding as members of it and recognise the autonomy of the will with its consequence, morality; whereas, if we conceive ourselves as under obligation, we consider ourselves as belonging to the world of sense and at the same time to the world of understanding.

The suspicion we raised earlier has now been cleared up. We suspected a hidden circle in our reasoning from freedom to autonomy, and from autonomy to the moral law. The concern was that we assumed freedom because of the moral law, only to derive that law from freedom afterward. If so, we could give no reason at all for the moral law. We could only present the law as an assumption that took for granted what we were trying to prove. Sympathetic readers would gladly accept that assumption, but we could never offer the law as a claim we could prove. For we now see that, when we think of ourselves as free, we place ourselves within the world of understanding. As members of that world, we recognize the will’s autonomy and its consequence, morality. By contrast, when we think of ourselves as under obligation, we regard ourselves as belonging to the world of sense. At the same time, we regard ourselves as belonging to the world of understanding.

Every rational being reckons himself qua intelligence as belonging to the world of understanding, and it is simply as an efficient cause belonging to that world that he calls his causality a will. On the other side he is also conscious of himself as a part of the world of sense in which his actions, which are mere appearances [phenomena] of that causality, are displayed; we cannot, however, discern how they are possible from this causality which we do not know; but instead of that, these actions as belonging to the sensible world must be viewed as determined by other phenomena, namely, desires and inclinations. If therefore I were only a member of the world of understanding, then all my actions would perfectly conform to the principle of autonomy of the pure will; if I were only a part of the world of sense, they would necessarily be assumed to conform wholly to the natural law of desires and inclinations, in other words, to the heteronomy of nature. (The former would rest on morality as the supreme principle, the latter on happiness.) Since, however, the world of understanding contains the foundation of the world of sense, and consequently of its laws also, and accordingly gives the law to my will (which belongs wholly to the world of understanding) directly, and must be conceived as doing so, it follows that, although on the one side I must regard myself as a being belonging to the world of sense, yet on the other side I must recognize myself as subject as an intelligence to the law of the world of understanding, i.e., to reason, which contains this law in the idea of freedom, and therefore as subject to the autonomy of the will: consequently I must regard the laws of the world of understanding as imperatives for me and the actions which conform to them as duties.

Every rational being sees himself, as an intelligence, as belonging to the world of understanding. Only as an efficient cause belonging to that world does he call his power to cause actions a will. At the same time, he knows himself as part of the world of sense. His actions appear in the world of sense as mere appearances [phenomena] of that power. We do not know this power itself, however, and cannot understand how it makes these actions possible. Instead, we must view these actions, as parts of the world of sense, as determined by other phenomena: desires and inclinations. If I belonged only to the world of understanding, all my actions would perfectly follow the pure will’s principle of autonomy. If I belonged only to the world of sense, my actions would necessarily be assumed to follow nature’s law of desires and inclinations entirely. In other words, my actions would follow the heteronomy of nature, or determination by nature rather than self-government. (The first would rest on morality as the highest principle, the second on happiness.) However, the world of understanding contains the foundation of the world of sense, and therefore of its laws too. The world of understanding therefore directly gives the law to my will, which belongs entirely to the world of understanding. We must also understand the world of understanding as giving this law. So, although I must regard myself as belonging to the world of sense, I must also recognize another obligation. As an intelligence, I am subject to the law of the world of understanding, that is, to reason. Reason contains this law in the idea of freedom. I must therefore recognize myself as subject to the autonomy of the will. Consequently, I must regard the laws of the world of understanding as commands for me. I must regard actions that follow these laws as duties.

And thus what makes categorical imperatives possible is this, that the idea of freedom makes me a member of an intelligible world, in consequence of which, if I were nothing else, all my actions would always conform to the autonomy of the will; but as I at the same time intuite myself as a member of the world of sense, they ought so to conform, and this categorical "ought" implies a synthetic a priori proposition, inasmuch as besides my will as affected by sensible desires there is added further the idea of the same will but as belonging to the world of the understanding, pure and practical of itself, which contains the supreme condition according to reason of the former will; precisely as to the intuitions of sense there are added concepts of the understanding which of themselves signify nothing but regular form in general and in this way synthetic a priori propositions become possible, on which all knowledge of physical nature rests.

Categorical imperatives are therefore possible because the idea of freedom makes me a member of an intelligible world. If I belonged only to that world, all my actions would always follow the autonomy of the will. But I also perceive myself as a member of the world of sense, so my actions ought to follow that autonomy. This categorical "ought" expresses a synthetic a priori proposition. For alongside my will as influenced by sensory desires, I also conceive of that same will as belonging to the world of understanding. In that world, the will is pure and practical in itself. According to reason, this pure will contains the highest condition governing my will as influenced by sensory desires. The same happens when concepts of the understanding are added to sensory intuitions. These concepts, taken by themselves, express nothing but regular form in general. In this way, synthetic a priori propositions become possible, and all knowledge of physical nature rests on them.

The practical use of common human reason confirms this reasoning. There is no one, not even the most consummate villain, provided only that he is otherwise accustomed to the use of reason, who, when we set before him examples of honesty of purpose, of steadfastness in following good maxims, of sympathy and general benevolence (even combined with great sacrifices of advantages and comfort), does not wish that he might also possess these qualities. Only on account of his inclinations and impulses he cannot attain this in himself, but at the same time he wishes to be free from such inclinations which are burdensome to himself. He proves by this that he transfers himself in thought with a will free from the impulses of the sensibility into an order of things wholly different from that of his desires in the field of the sensibility; since he cannot expect to obtain by that wish any gratification of his desires, nor any position which would satisfy any of his actual or supposable inclinations (for this would destroy the pre-eminence of the very idea which wrests that wish from him): he can only expect a greater intrinsic worth of his own person. This better person, however, he imagines himself to be when be transfers himself to the point of view of a member of the world of the understanding, to which he is involuntarily forced by the idea of freedom, i.e., of independence on determining causes of the world of sense; and from this point of view he is conscious of a good will, which by his own confession constitutes the law for the bad will that he possesses as a member of the world of sense- a law whose authority he recognizes while transgressing it. What he morally "ought" is then what he necessarily "would," as a member of the world of the understanding, and is conceived by him as an "ought" only inasmuch as he likewise considers himself as a member of the world of sense.

The practical use of ordinary human reason confirms this argument. Even the most hardened villain wishes he had certain qualities, provided he is otherwise accustomed to using reason. He feels this wish when shown examples of honest intentions, firm commitment to good principles, sympathy, and goodwill toward everyone. He wishes for these qualities even when they involve great sacrifices of personal advantage and comfort. Only his inclinations and impulses prevent him from developing these qualities. At the same time, he wishes to be free from those inclinations, which he himself finds burdensome. This wish shows that, in thought, he places himself in an entirely different order of things. He imagines his will free from the impulses of the senses, outside the realm of his sensory desires. For he cannot expect this wish to satisfy any desire. Nor can he expect any situation that would satisfy any inclination he actually has or could possibly have. Such an expectation would destroy the superiority of the very idea that draws this wish from him. He can expect only to have greater worth as a person, valuable in himself. He imagines himself as this better person when he adopts the standpoint of a member of the world of understanding. The idea of freedom inevitably forces him to adopt this standpoint. Freedom here means independence from the causes in the world of sense that determine his actions. From this standpoint, he is aware of a good will. By his own admission, this good will lays down the law for his bad will as a member of the world of sense. He recognizes this law’s authority even while breaking it. What he morally "ought" to do is therefore what he necessarily "would" do as a member of the world of understanding. He thinks of this action as an "ought" only because he also considers himself a member of the world of sense.

All men attribute to themselves freedom of will. Hence come all judgements upon actions as being such as ought to have been done, although they have not been done. However, this freedom is not a conception of experience, nor can it be so, since it still remains, even though experience shows the contrary of what on supposition of freedom are conceived as its necessary consequences. On the other side it is equally necessary that everything that takes place should be fixedly determined according to laws of nature. This necessity of nature is likewise not an empirical conception, just for this reason, that it involves the motion of necessity and consequently of a priori cognition. But this conception of a system of nature is confirmed by experience; and it must even be inevitably presupposed if experience itself is to be possible, that is, a connected knowledge of the objects of sense resting on general laws. Therefore freedom is only an idea of reason, and its objective reality in itself is doubtful; while nature is a concept of the understanding which proves, and must necessarily prove, its reality in examples of experience.

All people regard themselves as having free will. This belief underlies all judgments that actions ought to have been done, even though they were not done. Yet freedom is not a concept drawn from experience, nor can it be. The idea of freedom persists even when experience contradicts what we regard as its necessary consequences, assuming freedom exists. On the other hand, everything that happens must be firmly determined by the laws of nature. This necessity in nature is likewise not a concept drawn from experience. The reason is that the concept involves necessity and therefore a priori knowledge, or knowledge independent of experience. But experience confirms this concept of a system of nature. Indeed, we must presuppose such a system for experience itself to be possible. Experience means connected knowledge of the objects of our senses, based on general laws. Thus freedom is only an idea of reason, and whether freedom has objective reality in itself remains doubtful. Nature, by contrast, is a concept of the understanding whose reality is demonstrated, and must be demonstrated, through examples from experience.

There arises from this a dialectic of reason, since the freedom attributed to the will appears to contradict the necessity of nature, and placed between these two ways reason for speculative purposes finds the road of physical necessity much more beaten and more appropriate than that of freedom; yet for practical purposes the narrow footpath of freedom is the only one on which it is possible to make use of reason in our conduct; hence it is just as impossible for the subtlest philosophy as for the commonest reason of men to argue away freedom. Philosophy must then assume that no real contradiction will be found between freedom and physical necessity of the same human actions, for it cannot give up the conception of nature any more than that of freedom.

A conflict within reason arises from this, because the freedom attributed to the will seems to contradict nature’s necessity. Reason stands between these two paths. For theoretical purposes, the path of physical necessity is far better traveled and more suitable than the path of freedom. Yet for practical purposes, freedom’s narrow footpath is the only way to use reason to guide our conduct. So even the most subtle philosophy can no more argue away freedom than ordinary human reason can. Philosophy must therefore assume that freedom and physical necessity do not truly contradict each other in the same human actions. For philosophy can no more abandon the concept of nature than the concept of freedom.

Nevertheless, even though we should never be able to comprehend how freedom is possible, we must at least remove this apparent contradiction in a convincing manner. For if the thought of freedom contradicts either itself or nature, which is equally necessary, it must in competition with physical necessity be entirely given up.

Still, even if we can never understand how freedom is possible, we must at least convincingly resolve this apparent contradiction. For if freedom contradicts itself or nature (which is equally necessary), freedom must be abandoned entirely in favor of physical necessity.

It would, however, be impossible to escape this contradiction if the thinking subject, which seems to itself free, conceived itself in the same sense or in the very same relation when it calls itself free as when in respect of the same action it assumes itself to be subject to the law of nature. Hence it is an indispensable problem of speculative philosophy to show that its illusion respecting the contradiction rests on this, that we think of man in a different sense and relation when we call him free and when we regard him as subject to the laws of nature as being part and parcel of nature. It must therefore show that not only can both these very well co-exist, but that both must be thought as necessarily united in the same subject, since otherwise no reason could be given why we should burden reason with an idea which, though it may possibly without contradiction be reconciled with another that is sufficiently established, yet entangles us in a perplexity which sorely embarrasses reason in its theoretic employment. This duty, however, belongs only to speculative philosophy. The philosopher then has no option whether he will remove the apparent contradiction or leave it untouched; for in the latter case the theory respecting this would be bonum vacans, into the possession of which the fatalist would have a right to enter and chase all morality out of its supposed domain as occupying it without title.

Yet this contradiction would be inescapable under one condition. The thinking subject, which seems free to itself, would have to understand itself in the same sense or relation in both cases. In one case, the subject calls itself free. In the other, the subject assumes that the very same action is governed by the law of nature. Speculative philosophy must therefore explain why the apparent contradiction is an illusion. The illusion arises because we understand a person in different senses and relations in these two cases. We call the person free, yet also regard that person as part of nature and subject to its laws. Speculative philosophy must show not only that freedom and subjection to nature can coexist. It must also show that we must think of both as necessarily united in the same subject. Otherwise, we could give no reason for burdening reason with the idea of freedom. That idea may be compatible, without contradiction, with the well-established idea that we are subject to natural laws. Yet the idea of freedom entangles us in a difficulty that seriously troubles reason in its theoretical use. The duty to resolve this difficulty, however, belongs solely to speculative philosophy. The philosopher therefore has no choice about whether to resolve the apparent contradiction or leave it untouched. If left untouched, the theory concerning this matter would be bonum vacans, property belonging to no one. The fatalist would then have the right to take possession and drive all morality from its supposed domain. Morality would be occupying that domain without any right to it.

We cannot however as yet say that we are touching the bounds of practical philosophy. For the settlement of that controversy does not belong to it; it only demands from speculative reason that it should put an end to the discord in which it entangles itself in theoretical questions, so that practical reason may have rest and security from external attacks which might make the ground debatable on which it desires to build.

However, we cannot yet say that we are reaching the limits of practical philosophy. For settling that dispute is not the task of practical philosophy. It asks only that speculative reason resolve the conflicts it gets itself into over theoretical questions. The purpose is to give practical reason peace and security against outside attacks. Such attacks might call into question the ground on which practical reason seeks to build.

The claims to freedom of will made even by common reason are founded on the consciousness and the admitted supposition that reason is independent of merely subjectively determined causes which together constitute what belongs to sensation only and which consequently come under the general designation of sensibility. Man considering himself in this way as an intelligence places himself thereby in a different order of things and in a relation to determining grounds of a wholly different kind when on the one hand he thinks of himself as an intelligence endowed with a will, and consequently with causality, and when on the other he perceives himself as a phenomenon in the world of sense (as he really is also), and affirms that his causality is subject to external determination according to laws of nature. Now he soon becomes aware that both can hold good, nay, must hold good at the same time. For there is not the smallest contradiction in saying that a thing in appearance (belonging to the world of sense) is subject to certain laws, of which the very same as a thing or being in itself is independent, and that he must conceive and think of himself in this twofold way, rests as to the first on the consciousness of himself as an object affected through the senses, and as to the second on the consciousness of himself as an intelligence, i.e., as independent on sensible impressions in the employment of his reason (in other words as belonging to the world of understanding).

Even ordinary reason claims that the will is free. This claim rests on our awareness and accepted assumption that reason is independent of causes determined merely by subjective conditions. These causes together belong only to sensation and therefore fall under the general heading of sensibility. When a person considers himself as an intelligence, he places himself in a different order of things. He also sees himself as subject to an entirely different kind of determining grounds. On one hand, he thinks of himself as an intelligence with a will, and therefore with causal power. On the other, he perceives himself as an appearance in the world of sense, as he also really is. From this second standpoint, he holds that external conditions determine his causal activity according to the laws of nature. He soon recognizes that both views can hold true at once. Indeed, both must hold true at once. There is not the slightest contradiction here. A thing as it appears in the world of sense is subject to certain laws. Yet that very same thing, considered as a thing or being in itself, is independent of those laws. A person must conceive of himself in both ways. The first view rests on his awareness of himself as an object affected through the senses. The second rests on his awareness of himself as an intelligence. As an intelligence, he uses reason independently of sensory impressions. In other words, he belongs to the world of understanding.

Hence it comes to pass that man claims the possession of a will which takes no account of anything that comes under the head of desires and inclinations and, on the contrary, conceives actions as possible to him, nay, even as necessary which can only be done by disregarding all desires and sensible inclinations. The causality of such actions lies in him as an intelligence and in the laws of effects and actions [which depend] on the principles of an intelligible world, of which indeed he knows nothing more than that in it pure reason alone independent of sensibility gives the law; moreover since it is only in that world, as an intelligence, that he is his proper self (being as man only the appearance of himself), those laws apply to him directly and categorically, so that the incitements of inclinations and appetites (in other words the whole nature of the world of sense) cannot impair the laws of his volition as an intelligence. Nay, he does not even hold himself responsible for the former or ascribe them to his proper self, i.e., his will: he only ascribes to his will any indulgence which he might yield them if he allowed them to influence his maxims to the prejudice of the rational laws of the will.

This is why a person claims to have a will that takes no account of desires and inclinations. Instead, he sees certain actions as possible for him, and even necessary, though they require disregarding all desires and sensory inclinations. The cause of such actions lies in him as an intelligence. It also lies in laws governing effects and actions [which depend] on the principles of an intelligible world. He knows nothing more about that world than that pure reason alone, independent of the senses, gives its laws. Moreover, only in that world, as an intelligence, is he his true self. As a human being, he is only an appearance of himself. Those laws therefore apply to him directly and unconditionally. So the urges of inclinations and appetites cannot weaken the laws governing his will as an intelligence. These urges, in other words, encompass the whole nature of the sensory world. Indeed, he does not even hold himself responsible for those urges or attribute them to his true self, his will. He attributes to his will only whatever indulgence he might grant those urges. He would grant such indulgence if he let those urges influence his guiding principles against the rational laws of the will.

When practical reason thinks itself into a world of understanding, it does not thereby transcend its own limits, as it would if it tried to enter it by intuition or sensation. The former is only a negative thought in respect of the world of sense, which does not give any laws to reason in determining the will and is positive only in this single point that this freedom as a negative characteristic is at the same time conjoined with a (positive) faculty and even with a causality of reason, which we designate a will, namely a faculty of so acting that the principle of the actions shall conform to the essential character of a rational motive, i.e., the condition that the maxim have universal validity as a law. But were it to borrow an object of will, that is, a motive, from the world of understanding, then it would overstep its bounds and pretend to be acquainted with something of which it knows nothing. The conception of a world of the understanding is then only a point of view which reason finds itself compelled to take outside the appearances in order to conceive itself as practical, which would not be possible if the influences of the sensibility had a determining power on man, but which is necessary unless he is to be denied the consciousness of himself as an intelligence and, consequently, as a rational cause, energizing by reason, that is, operating freely. This thought certainly involves the idea of an order and a system of laws different from that of the mechanism of nature which belongs to the sensible world; and it makes the conception of an intelligible world necessary (that is to say, the whole system of rational beings as things in themselves). But it does not in the least authorize us to think of it further than as to its formal condition only, that is, the universality of the maxims of the will as laws, and consequently the autonomy of the latter, which alone is consistent with its freedom; whereas, on the contrary, all laws that refer to a definite object give heteronomy, which only belongs to laws of nature and can only apply to the sensible world.

When practical reason thinks of itself as belonging to a world of understanding, it does not exceed its limits. It would exceed those limits if it tried to enter that world through intuition or sensation. Merely thinking of itself this way is a negative thought about the world of sense. The world of sense gives reason no laws for determining the will. This thought has a positive side in only one respect. Freedom, though a negative characteristic, is joined to a positive capacity, indeed to reason’s power to cause actions. We call this power a will. A will is the capacity to act so that the principle guiding the action meets the essential requirement of a rational motive. That requirement is that the maxim be universally valid as a law. But suppose reason borrowed an object of the will, that is, a motive, from the world of understanding. Reason would then exceed its limits and claim knowledge of something about which it knows nothing. The idea of a world of understanding is therefore only a standpoint. Reason must adopt this standpoint outside appearances to conceive of itself as practical. Reason could not conceive of itself as practical if sensory influences determined human action. Yet this standpoint is necessary unless we deny human beings awareness of themselves as intelligences. Consequently, we would also deny their awareness of themselves as rational causes, acting through reason, that is, acting freely. This thought certainly implies an order and a system of laws different from nature’s mechanism in the world of sense. It also makes the idea of an intelligible world necessary. Such a world is the entire system of rational beings as things in themselves. But this thought does not authorize us to conceive of that world beyond its formal condition alone. That condition is that the will’s maxims have universal validity as laws, and consequently that the will is autonomous. Only this autonomy is consistent with the will’s freedom. By contrast, all laws referring to a definite object make the will subject to something outside itself, or heteronomous. Such dependence belongs only to laws of nature and can apply only to the world of sense.

But reason would overstep all its bounds if it undertook to explain how pure reason can be practical, which would be exactly the same problem as to explain how freedom is possible.

But reason would exceed all its limits if it tried to explain how pure reason can guide action. Explaining this would be exactly the same problem as explaining how freedom is possible.

For we can explain nothing but that which we can reduce to laws, the object of which can be given in some possible experience. But freedom is a mere idea, the objective reality of which can in no wise be shown according to laws of nature, and consequently not in any possible experience; and for this reason it can never be comprehended or understood, because we cannot support it by any sort of example or analogy. It holds good only as a necessary hypothesis of reason in a being that believes itself conscious of a will, that is, of a faculty distinct from mere desire (namely, a faculty of determining itself to action as an intelligence, in other words, by laws of reason independently on natural instincts). Now where determination according to laws of nature ceases, there all explanation ceases also, and nothing remains but defence, i.e., the removal of the objections of those who pretend to have seen deeper into the nature of things, and thereupon boldly declare freedom impossible. We can only point out to them that the supposed contradiction that they have discovered in it arises only from this, that in order to be able to apply the law of nature to human actions, they must necessarily consider man as an appearance: then when we demand of them that they should also think of him qua intelligence as a thing in itself, they still persist in considering him in this respect also as an appearance. In this view it would no doubt be a contradiction to suppose the causality of the same subject (that is, his will) to be withdrawn from all the natural laws of the sensible world. But this contradiction disappears, if they would only bethink themselves and admit, as is reasonable, that behind the appearances there must also lie at their root (although hidden) the things in themselves, and that we cannot expect the laws of these to be the same as those that govern their appearances.

For we can explain only what we can bring under laws whose objects can be given in some possible experience. But freedom is only an idea. Its objective reality cannot be shown through laws of nature, and therefore cannot be shown in any possible experience. For this reason, freedom can never be grasped or understood, because no example or analogy can support it. Freedom is valid only as a necessary assumption of reason for a being that believes itself conscious of a will. A will is a capacity distinct from mere desire. It is the capacity to determine its own actions as an intelligence. In other words, it acts through laws of reason, independently of natural instincts. Now, where determination by laws of nature ends, all explanation also ends. Nothing remains but to defend freedom by removing the objections of those who claim deeper insight into things. On the strength of that claim, these critics boldly declare freedom impossible. We can only point out that their supposed contradiction arises from the following confusion. To apply the law of nature to human actions, they must necessarily consider a human being as an appearance. We then ask them also to consider the human being, as an intelligence, as a thing in itself. Yet even in this respect, they persist in treating the human being as an appearance. From that viewpoint, there would indeed be a contradiction in supposing that the same person's causality escapes all natural laws of the world of the senses. That causality is the person's will. But the contradiction disappears if the critics reconsider and accept what it is reasonable to accept. Behind appearances, things in themselves must also exist as their foundation, although hidden. We cannot expect the laws of things in themselves to be the same as the laws governing their appearances.

The subjective impossibility of explaining the freedom of the will is identical with the impossibility of discovering and explaining an interest¹⁸ which man can take in the moral law. Nevertheless he does actually take an interest in it, the basis of which in us we call the moral feeling, which some have falsely assigned as the standard of our moral judgement, whereas it must rather be viewed as the subjective effect that the law exercises on the will, the objective principle of which is furnished by reason alone.

Our subjective inability to explain freedom of the will is the same as another inability. We cannot discover and explain an interest¹⁸ that humans can take in the moral law. Nevertheless, we do take an interest in the moral law. We call the basis of this interest within us “moral feeling.” Some have wrongly made moral feeling the standard for our moral judgment. Instead, moral feeling must be understood as the moral law’s subjective effect on the will. Reason alone supplies the will’s objective principle.

In order indeed that a rational being who is also affected through the senses should will what reason alone directs such beings that they ought to will, it is no doubt requisite that reason should have a power to infuse a feeling of pleasure or satisfaction in the fulfilment of duty, that is to say, that it should have a causality by which it determines the sensibility according to its own principles. But it is quite impossible to discern, i.e., to make it intelligible a priori, how a mere thought, which itself contains nothing sensible, can itself produce a sensation of pleasure or pain; for this is a particular kind of causality of which as of every other causality we can determine nothing whatever a priori; we must only consult experience about it. But as this cannot supply us with any relation of cause and effect except between two objects of experience, whereas in this case, although indeed the effect produced lies within experience, yet the cause is supposed to be pure reason acting through mere ideas which offer no object to experience, it follows that for us men it is quite impossible to explain how and why the universality of the maxim as a law, that is, morality, interests. This only is certain, that it is not because it interests us that it has validity for us (for that would be heteronomy and dependence of practical reason on sensibility, namely, on a feeling as its principle, in which case it could never give moral laws), but that it interests us because it is valid for us as men, inasmuch as it had its source in our will as intelligences, in other words, in our proper self, and what belongs to mere appearance is necessarily subordinated by reason to the nature of the thing in itself.

A rational being affected by the senses must be able to will what reason alone says such beings ought to will. For this to happen, reason must undoubtedly have the power to produce pleasure or satisfaction in fulfilling duty. In other words, reason must be able to shape our capacity for feeling according to its own principles. But we cannot understand a priori, or independently of experience, how a mere thought can produce pleasure or pain. The thought itself contains nothing sensory. This is a particular kind of causation. As with every other kind of causation, we can determine nothing about it a priori. We must rely entirely on experience. But experience can reveal a relationship between cause and effect only between two objects of experience. In this case, however, although the effect occurs within experience, the supposed cause is pure reason. Reason is supposed to act through mere ideas that present no object to experience. So we humans cannot explain how or why morality engages our interest. That is, we cannot explain our interest in a maxim's universality as a law. Only this is certain: morality is not valid for us because it interests us. That would mean heteronomy, making practical reason dependent on our capacity for feeling, with a feeling as its guiding principle. In that case, practical reason could never give moral laws. Rather, morality interests us because it is valid for us as human beings. Its source is our will as rational beings, in other words, our true self. And reason necessarily subordinates what belongs merely to appearance to the nature of the thing in itself.

The question then, "How a categorical imperative is possible," can be answered to this extent, that we can assign the only hypothesis on which it is possible, namely, the idea of freedom; and we can also discern the necessity of this hypothesis, and this is sufficient for the practical exercise of reason, that is, for the conviction of the validity of this imperative, and hence of the moral law; but how this hypothesis itself is possible can never be discerned by any human reason. On the hypothesis, however, that the will of an intelligence is free, its autonomy, as the essential formal condition of its determination, is a necessary consequence. Moreover, this freedom of will is not merely quite possible as a hypothesis (not involving any contradiction to the principle of physical necessity in the connexion of the phenomena of the sensible world) as speculative philosophy can show: but further, a rational being who is conscious of causality through reason, that is to say, of a will (distinct from desires), must of necessity make it practically, that is, in idea, the condition of all his voluntary actions. But to explain how pure reason can be of itself practical without the aid of any spring of action that could be derived from any other source, i.e., how the mere principle of the universal validity of all its maxims as laws (which would certainly be the form of a pure practical reason) can of itself supply a spring, without any matter (object) of the will in which one could antecedently take any interest; and how it can produce an interest which would be called purely moral; or in other words, how pure reason can be practical- to explain this is beyond the power of human reason, and all the labour and pains of seeking an explanation of it are lost.

The question, "How is a categorical imperative possible," can be answered only to this extent. We can identify the only assumption that makes such an imperative possible: the idea of freedom. We can also recognize why this assumption is necessary. For the practical use of reason, that is enough to establish conviction that this imperative, and therefore the moral law, is valid. But no human reason can ever understand how this assumption itself is possible. If, however, we assume that a rational being’s will is free, the will’s autonomy necessarily follows. This autonomy is the essential condition governing the form of the will’s decisions. Moreover, speculative philosophy can show that freedom of the will is a possible assumption. This assumption does not contradict the principle that physical necessity connects the appearances of the world we perceive through our senses. But freedom is more than a possible assumption. A rational being is conscious of causing actions through reason, that is, of having a will distinct from desires. Such a being must necessarily assume freedom as the condition of all their voluntary actions. This assumption is practical, meaning that the being must act under the idea of freedom. But how can pure reason guide action by itself, without any motive drawn from another source? Consider the principle that all the will’s maxims must be universally valid as laws. This principle would certainly give pure practical reason its form. But how can this principle alone provide a motive? How can it do so without any object of the will in which one could already take an interest? And how can it produce an interest that would be called purely moral? In other words, how can pure reason guide action? Explaining this lies beyond the power of human reason. All the effort and trouble spent seeking an explanation are wasted.

It is just the same as if I sought to find out how freedom itself is possible as the causality of a will. For then I quit the ground of philosophical explanation, and I have no other to go upon. I might indeed revel in the world of intelligences which still remains to me, but although I have an idea of it which is well founded, yet I have not the least knowledge of it, nor an I ever attain to such knowledge with all the efforts of my natural faculty of reason. It signifies only a something that remains over when I have eliminated everything belonging to the world of sense from the actuating principles of my will, serving merely to keep in bounds the principle of motives taken from the field of sensibility; fixing its limits and showing that it does not contain all in all within itself, but that there is more beyond it; but this something more I know no further. Of pure reason which frames this ideal, there remains after the abstraction of all matter, i.e., knowledge of objects, nothing but the form, namely, the practical law of the universality of the maxims, and in conformity with this conception of reason in reference to a pure world of understanding as a possible efficient cause, that is a cause determining the will. There must here be a total absence of springs; unless this idea of an intelligible world is itself the spring, or that in which reason primarily takes an interest; but to make this intelligible is precisely the problem that we cannot solve.

It is just as if I tried to discover how freedom itself is possible as the causality of a will. I would then leave the ground of philosophical explanation, with no other ground to stand on. I might indeed delight in the world of intelligences that still remains open to me. But although my idea of this world is well founded, I have no knowledge of it at all. Nor can I ever gain such knowledge through all the efforts of my natural power of reason. This idea means only something left over after I remove everything sensory from the principles that move my will. Its only role is to set limits on the principle of motives drawn from sensory experience. It shows that this principle does not contain everything within itself, and that something more lies beyond it. But I know nothing further about this something more. Pure reason forms this ideal. Once all content, meaning knowledge of objects, is removed, nothing remains of pure reason but its form. This form is the practical law requiring maxims to be universal. Along with this law remains the conception of reason as a possible cause that determines the will. This conception relates reason to a pure world of understanding. Here there must be no motivating impulses at all. The only exception would be if the idea of an intelligible world were itself the motivating impulse. Or this idea would have to be what reason primarily takes an interest in. But explaining how this is possible is precisely the problem we cannot solve.

Here now is the extreme limit of all moral inquiry, and it is of great importance to determine it even on this account, in order that reason may not on the one hand, to the prejudice of morals, seek about in the world of sense for the supreme motive and an interest comprehensible but empirical; and on the other hand, that it may not impotently flap its wings without being able to move in the (for it) empty space of transcendent concepts which we call the intelligible world, and so lose itself amidst chimeras. For the rest, the idea of a pure world of understanding as a system of all intelligences, and to which we ourselves as rational beings belong (although we are likewise on the other side members of the sensible world), this remains always a useful and legitimate idea for the purposes of rational belief, although all knowledge stops at its threshold, useful, namely, to produce in us a lively interest in the moral law by means of the noble ideal of a universal kingdom of ends in themselves (rational beings), to which we can belong as members then only when we carefully conduct ourselves according to the maxims of freedom as if they were laws of nature.

Here we reach the outermost limit of all moral inquiry. Identifying this limit is especially important for two reasons. First, reason must not undermine morality by searching the world of sense for the highest motive. Nor must reason seek there an interest that is understandable but empirical, based on experience. Second, reason must not helplessly flap its wings in the realm of transcendent concepts, unable to move forward. We call this realm the intelligible world, but for reason it is an empty space. Reason must not lose itself there among fantasies. Still, the idea of a pure world of understanding remains useful and legitimate for rational belief. We conceive of this world as a system of all rational beings. We ourselves belong to this system as rational beings, although we also belong to the world of sense. All knowledge stops at the threshold of this world of understanding. Nevertheless, the idea can awaken in us a strong interest in the moral law. It does so through the noble ideal of a universal kingdom of ends in themselves (rational beings). We can belong to this kingdom only when we carefully act according to the maxims of freedom. We must follow these maxims as if they were laws of nature.

The speculative employment of reason with respect to nature leads to the absolute necessity of some supreme cause of the world: the practical employment of reason with a view to freedom leads also to absolute necessity, but only of the laws of the actions of a rational being as such. Now it is an essential principle of reason, however employed, to push its knowledge to a consciousness of its necessity (without which it would not be rational knowledge). It is, however, an equally essential restriction of the same reason that it can neither discern the necessity of what is or what happens, nor of what ought to happen, unless a condition is supposed on which it is or happens or ought to happen. In this way, however, by the constant inquiry for the condition, the satisfaction of reason is only further and further postponed. Hence it unceasingly seeks the unconditionally necessary and finds itself forced to assume it, although without any means of making it comprehensible to itself, happy enough if only it can discover a conception which agrees with this assumption. It is therefore no fault in our deduction of the supreme principle of morality, but an objection that should be made to human reason in general, that it cannot enable us to conceive the absolute necessity of an unconditional practical law (such as the categorical imperative must be). It cannot be blamed for refusing to explain this necessity by a condition, that is to say, by means of some interest assumed as a basis, since the law would then cease to be a supreme law of reason. And thus while we do not comprehend the practical unconditional necessity of the moral imperative, we yet comprehend its incomprehensibility, and this is all that can be fairly demanded of a philosophy which strives to carry its principles up to the very limit of human reason.

When reason studies nature, it leads us to the absolute necessity of some supreme cause of the world. When reason considers freedom in guiding action, it also leads us to absolute necessity. But here, only the laws governing the actions of a rational being as such are absolutely necessary. However reason is used, one essential principle is to pursue knowledge until we recognize why that knowledge is necessary. Without this awareness of necessity, our knowledge would not be rational. Yet reason also has an equally essential limit. Reason cannot understand why anything exists, happens, or ought to happen necessarily unless we assume a condition. That condition must explain why the thing exists, happens, or ought to happen. But continually asking for the condition only postpones reason's satisfaction further and further. So reason constantly seeks what is necessary without any conditions and finds itself forced to assume such unconditional necessity. Yet reason has no way to make this necessity understandable to itself. Reason is fortunate if it can even find a concept consistent with this assumption. Our inability to explain the absolute necessity of an unconditional practical law is therefore no flaw in our argument. Such a law, governing action without conditions, is what the categorical imperative must be. The objection concerns human reason in general, not our derivation of the supreme principle of morality. Nor can reason be blamed for refusing to explain this necessity through a condition, meaning some underlying interest. Such an explanation would mean that the law was no longer a supreme law of reason. So we do not understand why the moral imperative is unconditionally necessary for action. Yet we do understand why that necessity cannot be understood. This is all we can fairly demand of a philosophy that takes its principles to the limits of human reason.