Skeptical Doubts About How the Mind Works

20. All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, Relations of Ideas, and Matters of Fact. Of the first kind are the sciences of Geometry, Algebra, and Arithmetic; and in short, every affirmation which is either intuitively or demonstratively certain. That the square of the hypothenuse is equal to the square of the two sides, is a proposition which expresses a relation between these figures. That three times five is equal to the half of thirty, expresses a relation between these numbers. Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe. Though there never were a circle or triangle in nature, the truths demonstrated by Euclid would for ever retain their certainty and evidence.

20. All objects of human reason or inquiry may naturally be divided into two kinds: Relations of Ideas and Matters of Fact. The first kind includes the sciences of Geometry, Algebra, and Arithmetic. It also includes every statement that is intuitively or demonstrably certain. The square of the hypotenuse is equal to the sum of the squares of the other two sides expresses a relation between these figures. Three times five is equal to half of thirty expresses a relation between these numbers. Statements of this kind can be discovered by thought alone. They do not depend on anything that exists anywhere in the universe. Though there had never been a circle or triangle in nature, the truths demonstrated by Euclid would remain certain and evident forever.

21. Matters of fact, which are the second objects of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a like nature with the foregoing. The contrary of every matter of fact is still possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinctness, as if ever so conformable to reality. That the sun will not rise to-morrow is no less intelligible a proposition, and implies no more contradiction than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply a contradiction, and could never be distinctly conceived by the mind.

21. Matters of fact are the second objects of human reason. We do not establish them in the same way as the first. Nor is our evidence for their truth, however strong, of the same kind as the evidence described above. The opposite of every matter of fact is still possible because it can never imply a contradiction. The mind can conceive it just as easily and clearly as if it fully agreed with reality. That the sun will not rise tomorrow is just as intelligible a proposition and implies no more contradiction than the statement that it will rise. We would therefore try in vain to demonstrate that the sun will not rise tomorrow is false. If that proposition were demonstrably false, it would imply a contradiction. The mind could then never conceive it clearly.

It may, therefore, be a subject worthy of curiosity, to enquire what is the nature of that evidence which assures us of any real existence and matter of fact, beyond the present testimony of our senses, or the records of our memory. This part of philosophy, it is observable, has been little cultivated, either by the ancients or moderns; and therefore our doubts and errors, in the prosecution of so important an enquiry, may be the more excusable; while we march through such difficult paths without any guide or direction. They may even prove useful, by exciting curiosity, and destroying that implicit faith and security, which is the bane of all reasoning and free enquiry. The discovery of defects in the common philosophy, if any such there be, will not, I presume, be a discouragement, but rather an incitement, as is usual, to attempt something more full and satisfactory than has yet been proposed to the public.

It may therefore be worth asking what kind of evidence assures us of real existence and matters of fact beyond what our senses tell us now or what we remember. Both ancient and modern thinkers have paid little attention to this part of philosophy. So our doubts and errors in pursuing such an important inquiry may be more excusable. We are, after all, making our way along difficult paths without any guide or direction. Our doubts and errors may even prove useful. They may stir curiosity and undermine the unquestioning faith and confidence that ruin all reasoning and free inquiry. If we discover any defects in accepted philosophy, I presume the discovery will not discourage us. Rather, as usual, it will encourage us to attempt something more complete and satisfactory than anything yet offered to the public.

22. All reasonings concerning matter of fact seem to be founded on the relation of Cause and Effect. By means of that relation alone we can go beyond the evidence of our memory and senses. If you were to ask a man, why he believes any matter of fact, which is absent; for instance, that his friend is in the country, or in France; he would give you a reason; and this reason would be some other fact; as a letter received from him, or the knowledge of his former resolutions and promises. A man finding a watch or any other machine in a desert island, would conclude that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious. The hearing of an articulate voice and rational discourse in the dark assures us of the presence of some person: Why? because these are the effects of the human make and fabric, and closely connected with it. If we anatomize all the other reasonings of this nature, we shall find that they are founded on the relation of cause and effect, and that this relation is either near or remote, direct or collateral. Heat and light are collateral effects of fire, and the one effect may justly be inferred from the other.

22. All reasoning about matters of fact seems to be based on the relation of Cause and Effect. Only through this relation can we go beyond the evidence of our memory and senses. Suppose you ask a man why he believes a fact he cannot presently observe. For example, he might believe that his friend is in the country or in France. He would give you a reason, and that reason would be another fact. It might be a letter from his friend or knowledge of his friend’s earlier decisions and promises. A man who found a watch or any other machine on a desert island would conclude that people had once been there. All our reasoning about facts is of the same kind. In every case, we assume a connection between the present fact and the fact inferred from it. If nothing connected them, the inference would be entirely uncertain. Hearing a clear voice and sensible speech in the dark assures us that someone is present. Why? Because these are effects of the human body and its structure, and are closely connected with it. If we examine all other reasoning of this kind, we find that it rests on cause and effect. This relation may be close or distant, direct or indirect. Heat and light are both effects of fire, and either effect may reasonably be inferred from the other.

23. If we would satisfy ourselves, therefore, concerning the nature of that evidence, which assures us of matters of fact, we must enquire how we arrive at the knowledge of cause and effect.

23. So, if we want to understand the evidence that assures us of matters of fact, we must investigate how we come to know cause and effect.

I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other. Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects. Adam, though his rational faculties be supposed, at the very first, entirely perfect, could not have inferred from the fluidity and transparency of water that it would suffocate him, or from the light and warmth of fire that it would consume him. No object ever discovers, by the qualities which appear to the senses, either the causes which produced it, or the effects which will arise from it; nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matter of fact.

I venture to state a general rule that admits of no exceptions. Knowledge of this relation is never gained through a priori reasoning. It comes entirely from experience, when we find that particular objects are constantly found together. Suppose an object is presented to someone with the strongest natural reasoning abilities. If the object is entirely new to that person, no examination of its perceptible qualities, however careful, will reveal its causes or effects. Even if Adam’s reasoning abilities were perfect from the very beginning, he could not have inferred that water would suffocate him from its fluidity and transparency. Nor could he have inferred that fire would consume him from its light and warmth. No object ever reveals, through the qualities we perceive with our senses, either the causes that produced it or the effects that will result from it. Nor can reason, without the help of experience, ever lead us to any conclusion about real existence and matters of fact.

24. This proposition, that causes and effects are discoverable, not by reason but by experience, will readily be admitted with regard to such objects, as we remember to have once been altogether unknown to us; since we must be conscious of the utter inability, which we then lay under, of foretelling what would arise from them. Present two smooth pieces of marble to a man who has no tincture of natural philosophy; he will never discover that they will adhere together in such a manner as to require great force to separate them in a direct line, while they make so small a resistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that the explosion of gunpowder, or the attraction of a loadstone, could ever be discovered by arguments a priori. In like manner, when an effect is supposed to depend upon an intricate machinery or secret structure of parts, we make no difficulty in attributing all our knowledge of it to experience. Who will assert that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or a tiger?

24. The proposition that causes and effects are discovered not by reason but by experience is readily accepted for things we remember once knowing nothing about. We must recognize that, at the time, we were completely unable to predict what would result from them. Show two smooth pieces of marble to a man with no knowledge of natural philosophy. He will never discover that they stick together so firmly that great force is needed to pull them straight apart. Yet they offer very little resistance when pushed sideways. Events that bear little resemblance to the usual course of nature are also readily admitted to be known only through experience. No one imagines that the explosion of gunpowder or the attraction of a lodestone could ever be discovered by a priori reasoning. Similarly, an effect may be thought to depend on intricate machinery or a hidden arrangement of parts. In such cases, we readily attribute all our knowledge of the effect to experience. Who will claim to know the ultimate reason why milk or bread nourishes a man, but not a lion or a tiger?

But the same truth may not appear, at first sight, to have the same evidence with regard to events, which have become familiar to us from our first appearance in the world, which bear a close analogy to the whole course of nature, and which are supposed to depend on the simple qualities of objects, without any secret structure of parts. We are apt to imagine that we could discover these effects by the mere operation of our reason, without experience. We fancy, that were we brought on a sudden into this world, we could at first have inferred that one Billiard-ball would communicate motion to another upon impulse; and that we needed not to have waited for the event, in order to pronounce with certainty concerning it. Such is the influence of custom, that, where it is strongest, it not only covers our natural ignorance, but even conceals itself, and seems not to take place, merely because it is found in the highest degree.

But this same truth may not seem as evident when it concerns events familiar to us since we first entered the world. These events closely resemble the whole course of nature. They are also thought to depend on the simple qualities of objects, without any hidden arrangement of parts. We are inclined to think that reason alone could reveal these effects to us, without experience. We imagine that, if we were suddenly brought into this world, we could immediately infer what happens when one billiard ball strikes another. We think we could infer that the first ball would set the second in motion. And we think we would not need to wait for the event before stating with certainty what would happen. Such is the influence of habit that, where it is strongest, it not only hides our natural ignorance but also hides itself. Simply because habit is strongest there, it seems to have no influence at all.

25. But to convince us that all the laws of nature, and all the operations of bodies without exception, are known only by experience, the following reflections may, perhaps, suffice. Were any object presented to us, and were we required to pronounce concerning the effect, which will result from it, without consulting past observation; after what manner, I beseech you, must the mind proceed in this operation? It must invent or imagine some event, which it ascribes to the object as its effect; and it is plain that this invention must be entirely arbitrary. The mind can never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and consequently can never be discovered in it. Motion in the second Billiard-ball is a quite distinct event from motion in the first; nor is there anything in the one to suggest the smallest hint of the other. A stone or piece of metal raised into the air, and left without any support, immediately falls: but to consider the matter a priori, is there anything we discover in this situation which can beget the idea of a downward, rather than an upward, or any other motion, in the stone or metal? And as the first imagination or invention of a particular effect, in all natural operations, is arbitrary, where we consult not experience; so must we also esteem the supposed tie or connexion between the cause and effect, which binds them together, and renders it impossible that any other effect could result from the operation of that cause. When I see, for instance, a Billiard-ball moving in a straight line towards another; even suppose motion in the second ball should by accident be suggested to me, as the result of their contact or impulse; may I not conceive, that a hundred different events might as well follow from that cause? May not both these balls remain at absolute rest? May not the first ball return in a straight line, or leap off from the second in any line or direction? All these suppositions are consistent and conceivable. Why then should we give the preference to one, which is no more consistent or conceivable than the rest? All our reasonings a priori will never be able to show us any foundation for this preference.

25. The following reflections may perhaps be enough to convince us that all laws of nature and all operations of bodies, without exception, are known only through experience. Suppose an object were presented to us. Suppose we were asked to state what effect it would produce, without consulting past observations. How, I ask, would the mind proceed? It would have to invent or imagine some event and assign it to the object as its effect. And clearly, this invention would have to be entirely arbitrary. No matter how closely the mind examined the supposed cause, it could never find the effect in it. For the effect is completely different from the cause, and so it can never be discovered in it. The motion of the second billiard ball is a completely distinct event from the motion of the first. There is nothing in the motion of the first ball to suggest the slightest hint of the second ball’s motion. A stone or piece of metal raised into the air and left without support immediately falls. But consider the matter a priori. Is there anything in this situation that could give us the idea of downward motion, rather than upward motion or any other motion, in the stone or metal? Without consulting experience, our first imagining or invention of a particular effect in any natural operation is arbitrary. We must also regard the supposed link between cause and effect as arbitrary. This link supposedly binds them together and makes it impossible for any other effect to result from that cause. For example, I see a billiard ball moving in a straight line toward another. Suppose even that it happens to occur to me that the second ball will move when the first strikes it. Can I not also imagine that a hundred different events might follow from the same cause? Could both balls not remain completely at rest? Could the first ball not return in a straight line, or bounce off the second in any direction? All these possibilities are consistent and conceivable. Why, then, should we prefer one that is no more consistent or conceivable than the rest? All our a priori reasoning can never show us any basis for this preference.

In a word, then, every effect is a distinct event from its cause. It could not, therefore, be discovered in the cause, and the first invention or conception of it, a priori, must be entirely arbitrary. And even after it is suggested, the conjunction of it with the cause must appear equally arbitrary; since there are always many other effects, which, to reason, must seem fully as consistent and natural. In vain, therefore, should we pretend to determine any single event, or infer any cause or effect, without the assistance of observation and experience.

In short, every effect is a distinct event from its cause. So the effect cannot be discovered in the cause. Any first idea of the effect formed a priori must therefore be entirely arbitrary. Even when an effect is suggested, its connection with the cause must seem equally arbitrary. For there are always many other effects that reason must find just as consistent and natural. We cannot, therefore, determine any single event or infer any cause or effect without the help of observation and experience.

26. Hence we may discover the reason why no philosopher, who is rational and modest, has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power, which produces any single effect in the universe. It is confessed, that the utmost effort of human reason is to reduce the principles, productive of natural phenomena, to a greater simplicity, and to resolve the many particular effects into a few general causes, by means of reasonings from analogy, experience, and observation. But as to the causes of these general causes, we should in vain attempt their discovery; nor shall we ever be able to satisfy ourselves, by any particular explication of them. These ultimate springs and principles are totally shut up from human curiosity and enquiry. Elasticity, gravity, cohesion of parts, communication of motion by impulse; these are probably the ultimate causes and principles which we shall ever discover in nature; and we may esteem ourselves sufficiently happy, if, by accurate enquiry and reasoning, we can trace up the particular phenomena to, or near to, these general principles. The most perfect philosophy of the natural kind only staves off our ignorance a little longer: as perhaps the most perfect philosophy of the moral or metaphysical kind serves only to discover larger portions of it. Thus the observation of human blindness and weakness is the result of all philosophy, and meets us at every turn, in spite of our endeavours to elude or avoid it.

26. This helps explain why no rational and modest philosopher has ever claimed to identify the ultimate cause of any natural process. Nor has such a philosopher claimed to show exactly how the power producing any single effect in the universe works. It is acknowledged that human reason can do no more than simplify the principles that produce natural phenomena. It can trace many particular effects to a few general causes, by reasoning from analogy, experience, and observation. But we would search in vain for the causes of these general causes. We will never be able to explain them in a way that satisfies us. These ultimate forces and principles are completely beyond human curiosity and inquiry. Elasticity, gravity, the cohesion of parts, and the transfer of motion by impact are probably the ultimate causes and principles we will ever discover in nature. We may count ourselves fortunate enough if careful inquiry and reasoning can trace particular phenomena to or near these general principles. Even the most perfect natural philosophy only puts off our ignorance a little longer. And perhaps the most perfect moral or metaphysical philosophy only reveals more of our ignorance. Thus all philosophy leads us to recognize human blindness and weakness. We meet them at every turn, despite our efforts to escape or avoid them.

27. Nor is geometry, when taken into the assistance of natural philosophy, ever able to remedy this defect, or lead us into the knowledge of ultimate causes, by all that accuracy of reasoning for which it is so justly celebrated. Every part of mixed mathematics proceeds upon the supposition that certain laws are established by nature in her operations; and abstract reasonings are employed, either to assist experience in the discovery of these laws, or to determine their influence in particular instances, where it depends upon any precise degree of distance and quantity. Thus, it is a law of motion, discovered by experience, that the moment or force of any body in motion is in the compound ratio or proportion of its solid contents and its velocity; and consequently, that a small force may remove the greatest obstacle or raise the greatest weight, if, by any contrivance or machinery, we can increase the velocity of that force, so as to make it an overmatch for its antagonist. Geometry assists us in the application of this law, by giving us the just dimensions of all the parts and figures which can enter into any species of machine; but still the discovery of the law itself is owing merely to experience, and all the abstract reasonings in the world could never lead us one step towards the knowledge of it. When we reason a priori, and consider merely any object or cause, as it appears to the mind, independent of all observation, it never could suggest to us the notion of any distinct object, such as its effect; much less, show us the inseparable and inviolable connexion between them. A man must be very sagacious who could discover by reasoning that crystal is the effect of heat, and ice of cold, without being previously acquainted with the operation of these qualities.

27. Geometry cannot remedy this defect or reveal ultimate causes when applied to natural philosophy. This is true despite the accuracy of reasoning for which geometry is so rightly celebrated. Every branch of mixed mathematics assumes that nature operates according to certain established laws. Abstract reasoning helps us discover these laws through experience. It also helps us determine their effects in particular cases, when those effects depend on precise distances and quantities. For example, experience has revealed a law of motion. The momentum or force of a moving body is proportional to both its volume and its velocity. As a result, a small force may move the greatest obstacle or lift the greatest weight. It can do so if some device or machine increases its velocity enough to overcome the opposing force. Geometry helps us apply this law by giving us the precise dimensions of every part and shape used in any kind of machine. But the discovery of the law itself is due entirely to experience. All the abstract reasoning in the world could never bring us one step closer to knowing it. When we reason a priori, we consider an object or cause as it appears to the mind, independently of all observation. The object or cause could never suggest to us the idea of a distinct object, such as its effect. Much less could it show us an inseparable and unbreakable connection between cause and effect. A person would have to be very shrewd to discover by reasoning that heat produces crystal and cold produces ice. This would be so if the person had not already observed how heat and cold work.

28. But we have not yet attained any tolerable satisfaction with regard to the question first proposed. Each solution still gives rise to a new question as difficult as the foregoing, and leads us on to farther enquiries. When it is asked, What is the nature of all our reasonings concerning matter of fact? the proper answer seems to be, that they are founded on the relation of cause and effect. When again it is asked, What is the foundation of all our reasonings and conclusions concerning that relation? it may be replied in one word, Experience. But if we still carry on our sifting humour, and ask, What is the foundation of all conclusions from experience? this implies a new question, which may be of more difficult solution and explication. Philosophers, that give themselves airs of superior wisdom and sufficiency, have a hard task when they encounter persons of inquisitive dispositions, who push them from every corner to which they retreat, and who are sure at last to bring them to some dangerous dilemma. The best expedient to prevent this confusion, is to be modest in our pretensions; and even to discover the difficulty ourselves before it is objected to us. By this means, we may make a kind of merit of our very ignorance.

28. But we have not yet reached a satisfactory answer to the original question. Each answer raises a new question as difficult as the last and leads us to further inquiry. When we ask, What is the nature of all our reasoning about matters of fact? the proper answer seems to be that it rests on cause and effect. When we ask, What is the basis of all our reasoning and conclusions about cause and effect? the answer is one word: Experience. But if we keep probing and ask, What is the basis of all conclusions drawn from experience? we raise a new question. This question may be more difficult to answer and explain. Philosophers who pretend to superior wisdom and competence have a hard time when they meet inquisitive people. Such people pursue them into every corner where they take refuge and are sure to bring them to some dangerous dilemma. The best way to avoid this confusion is to make modest claims. We should even discover the difficulty ourselves before someone raises it as an objection. In this way, we may make something of a virtue of our very ignorance.

I shall content myself, in this section, with an easy task, and shall pretend only to give a negative answer to the question here proposed. I say then, that, even after we have experience of the operations of cause and effect, our conclusions from that experience are not founded on reasoning, or any process of the understanding. This answer we must endeavour both to explain and to defend.

In this section, I will take on an easy task. I will only attempt to give a negative answer to the question posed here. My answer is that our conclusions from experience are not based on reasoning or any process of understanding. This is true even after we have experienced how cause and effect operate. I must now try to explain and defend this answer.

29. It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us those powers and principles on which the influence of those objects entirely depends. Our senses inform us of the colour, weight, and consistence of bread; but neither sense nor reason can ever inform us of those qualities which fit it for the nourishment and support of a human body. Sight or feeling conveys an idea of the actual motion of bodies; but as to that wonderful force or power, which would carry on a moving body for ever in a continued change of place, and which bodies never lose but by communicating it to others; of this we cannot form the most distant conception. But notwithstanding this ignorance of natural powers⁶ and principles, we always presume, when we see like sensible qualities, that they have like secret powers, and expect that effects, similar to those which we have experienced, will follow from them. If a body of like colour and consistence with that bread, which we have formerly eat, be presented to us, we make no scruple of repeating the experiment, and foresee, with certainty, like nourishment and support. Now this is a process of the mind or thought, of which I would willingly know the foundation. It is allowed on all hands that there is no known connexion between the sensible qualities and the secret powers; and consequently, that the mind is not led to form such a conclusion concerning their constant and regular conjunction, by anything which it knows of their nature. As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, nourished me; that is, a body of such sensible qualities was, at that time, endued with such secret powers: but does it follow, that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary. At least, it must be acknowledged that there is here a consequence drawn by the mind; that there is a certain step taken; a process of thought, and an inference, which wants to be explained. These two propositions are far from being the same, I have found that such an object has always been attended with such an effect, and I foresee, that other objects, which are, in appearance, similar, will be attended with similar effects. I shall allow, if you please, that the one proposition may justly be inferred from the other: I know, in fact, that it always is inferred. But if you insist that the inference is made by a chain of reasoning, I desire you to produce that reasoning. The connexion between these propositions is not intuitive. There is required a medium, which may enable the mind to draw such an inference, if indeed it be drawn by reasoning and argument. What that medium is, I must confess, passes my comprehension; and it is incumbent on those to produce it, who assert that it really exists, and is the origin of all our conclusions concerning matter of fact.

29. We must surely admit that nature keeps us far from all her secrets. She lets us know only a few surface qualities of objects. She hides the powers and principles on which their effects entirely depend. Our senses tell us the color, weight, and consistency of bread. But neither our senses nor reason can tell us what qualities make bread capable of nourishing and sustaining the human body. Sight or touch gives us an idea of the actual motion of bodies. But we cannot form even the faintest idea of the remarkable force or power that would keep a moving body changing place forever. Bodies lose this power only by passing it on to others. Despite our ignorance of natural powers⁶ and principles, we always assume that similar sensory qualities go with similar hidden powers. We expect effects like those we have experienced before. Suppose we are offered something with the same color and consistency as bread we have eaten before. We do not hesitate to try it again, and we confidently expect it to nourish and sustain us in the same way. This is a process of the mind, and I would like to know its basis. Everyone agrees that there is no known connection between sensory qualities and hidden powers. So nothing we know about their nature leads us to conclude that they are constantly and regularly joined. Past experience can give us direct and certain information only about the particular objects and time it covered. But why should we extend this experience to future times and other objects? For all we know, those objects may be similar only in appearance. This is the main question I want to press. The bread I ate before nourished me. In other words, something with those sensory qualities had those hidden powers at that time. But does it follow that other bread must nourish me at another time? Must similar sensory qualities always go with similar hidden powers? The conclusion does not seem necessary at all. At least, we must admit that the mind draws a conclusion here. It takes a step, following a process of thought and making an inference that needs to be explained. These two statements are far from identical: I have found that an object of this kind has always had a certain effect and I foresee that other objects, similar in appearance, will have similar effects. If you wish, I will grant that the second statement may rightly be inferred from the first. In fact, I know that we always make this inference. But if you insist that we make it through a chain of reasoning, please show me that reasoning. The connection between these statements is not intuitive. If reasoning and argument do lead the mind to draw this inference, some intermediate step must make it possible. I confess that I cannot understand what that step might be. Those who claim that it exists, and that it is the source of all our conclusions about matters of fact, must show what it is.

30. This negative argument must certainly, in process of time, become altogether convincing, if many penetrating and able philosophers shall turn their enquiries this way and no one be ever able to discover any connecting proposition or intermediate step, which supports the understanding in this conclusion. But as the question is yet new, every reader may not trust so far to his own penetration, as to conclude, because an argument escapes his enquiry, that therefore it does not really exist. For this reason it may be requisite to venture upon a more difficult task; and enumerating all the branches of human knowledge, endeavour to show that none of them can afford such an argument.

30. This negative argument must certainly become completely convincing over time, if many insightful and capable philosophers investigate the question. It will do so if none of them can ever discover a connecting proposition or intermediate step that supports the mind in reaching this conclusion. But the question is still new. Not every reader may trust their own insight enough to conclude that an argument does not exist simply because they cannot find it. For this reason, it may be necessary to attempt a more difficult task. We must list all the branches of human knowledge and try to show that none of them can provide such an argument.

All reasonings may be divided into two kinds, namely, demonstrative reasoning, or that concerning relations of ideas, and moral reasoning, or that concerning matter of fact and existence. That there are no demonstrative arguments in the case seems evident; since it implies no contradiction that the course of nature may change, and that an object, seemingly like those which we have experienced, may be attended with different or contrary effects. May I not clearly and distinctly conceive that a body, falling from the clouds, and which, in all other respects, resembles snow, has yet the taste of salt or feeling of fire? Is there any more intelligible proposition than to affirm, that all the trees will flourish in December and January, and decay in May and June? Now whatever is intelligible, and can be distinctly conceived, implies no contradiction, and can never be proved false by any demonstrative argument or abstract reasoning priori.

All reasoning can be divided into two kinds. The first is demonstrative reasoning, which concerns relations between ideas. The second is moral reasoning, which concerns matters of fact and existence. There seem to be no demonstrative arguments in this case. This is because there is no contradiction in supposing that the course of nature may change. Nor is there a contradiction in supposing that an object resembling those we have experienced may have different or opposite effects. Can I not clearly imagine something falling from the clouds that resembles snow in every other respect, yet tastes salty or feels like fire? Is any statement more intelligible than saying that all trees will flourish in December and January, and decay in May and June? Whatever is intelligible and can be clearly imagined involves no contradiction. It can never be proved false by demonstrative argument or abstract reasoning a priori.

If we be, therefore, engaged by arguments to put trust in past experience, and make it the standard of our future judgement, these arguments must be probable only, or such as regard matter of fact and real existence, according to the division above mentioned. But that there is no argument of this kind, must appear, if our explication of that species of reasoning be admitted as solid and satisfactory. We have said that all arguments concerning existence are founded on the relation of cause and effect; that our knowledge of that relation is derived entirely from experience; and that all our experimental conclusions proceed upon the supposition that the future will be conformable to the past. To endeavour, therefore, the proof of this last supposition by probable arguments, or arguments regarding existence, must be evidently going in a circle, and taking that for granted, which is the very point in question.

If arguments lead us to trust past experience and use it as the standard for future judgments, they must be probable arguments. As noted in the division above, these are arguments about matters of fact and real existence. But if our explanation of this kind of reasoning is sound and satisfactory, there can be no such argument. We have said that all arguments about existence are based on cause and effect. Our knowledge of cause and effect comes entirely from experience. And all our conclusions from experience assume that the future will resemble the past. So trying to prove this last assumption through probable arguments, or arguments about existence, is clearly circular. It takes for granted the very point in question.

31. In reality, all arguments from experience are founded on the similarity which we discover among natural objects, and by which we are induced to expect effects similar to those which we have found to follow from such objects. And though none but a fool or madman will ever pretend to dispute the authority of experience, or to reject that great guide of human life, it may surely be allowed a philosopher to have so much curiosity at least as to examine the principle of human nature, which gives this mighty authority to experience, and makes us draw advantage from that similarity which nature has placed among different objects. From causes which appear similar we expect similar effects. This is the sum of all our experimental conclusions. Now it seems evident that, if this conclusion were formed by reason, it would be as perfect at first, and upon one instance, as after ever so long a course of experience. But the case is far otherwise. Nothing so like as eggs; yet no one, on account of this appearing similarity, expects the same taste and relish in all of them. It is only after a long course of uniform experiments in any kind, that we attain a firm reliance and security with regard to a particular event. Now where is that process of reasoning which, from one instance, draws a conclusion, so different from that which it infers from a hundred instances that are nowise different from that single one? This question I propose as much for the sake of information, as with an intention of raising difficulties. I cannot find, I cannot imagine any such reasoning. But I keep my mind still open to instruction, if any one will vouchsafe to bestow it on me.

31. In fact, all arguments from experience rest on similarities we discover among natural objects. These similarities lead us to expect effects like those we have found such objects to produce. Only a fool or a madman would dispute the authority of experience or reject this great guide to human life. Still, a philosopher may surely be curious enough to examine the principle of human nature that gives experience such authority. This principle also lets us benefit from the similarities nature has placed among different objects. We expect similar effects from causes that appear similar. This sums up all the conclusions we draw from experience. Now, if reason led us to this conclusion, the conclusion would be as complete after one instance as after a long course of experience. But the situation is quite different. Nothing is more alike than eggs. Yet no one expects all eggs to have the same taste and flavor simply because they look alike. Only after a long series of consistent observations of any kind do we become confident that a particular event will occur. What reasoning, then, leads us to draw such different conclusions from one instance and from a hundred instances that are no different from it? I ask this question as much to learn as to raise difficulties. I cannot find or imagine any such reasoning. But I remain open to learning, if anyone is willing to instruct me.

32. Should it be said that, from a number of uniform experiments, we infer a connexion between the sensible qualities and the secret powers; this, I must confess, seems the same difficulty, couched in different terms. The question still recurs, on what process of argument this inference is founded? Where is the medium, the interposing ideas, which join propositions so very wide of each other? It is confessed that the colour, consistence, and other sensible qualities of bread appear not, of themselves, to have any connexion with the secret powers of nourishment and support. For otherwise we could infer these secret powers from the first appearance of these sensible qualities, without the aid of experience; contrary to the sentiment of all philosophers, and contrary to plain matter of fact. Here, then, is our natural state of ignorance with regard to the powers and influence of all objects. How is this remedied by experience? It only shows us a number of uniform effects, resulting from certain objects, and teaches us that those particular objects, at that particular time, were endowed with such powers and forces. When a new object, endowed with similar sensible qualities, is produced, we expect similar powers and forces, and look for a like effect. From a body of like colour and consistence with bread we expect like nourishment and support. But this surely is a step or progress of the mind, which wants to be explained. When a man says, I have found, in all past instances, such sensible qualities conjoined with such secret powers: And when he says, Similar sensible qualities will always be conjoined with similar secret powers, he is not guilty of a tautology, nor are these propositions in any respect the same. You say that the one proposition is an inference from the other. But you must confess that the inference is not intuitive; neither is it demonstrative: Of what nature is it, then? To say it is experimental, is begging the question. For all inferences from experience suppose, as their foundation, that the future will resemble the past, and that similar powers will be conjoined with similar sensible qualities. If there be any suspicion that the course of nature may change, and that the past may be no rule for the future, all experience becomes useless, and can give rise to no inference or conclusion. It is impossible, therefore, that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded on the supposition of that resemblance. Let the course of things be allowed hitherto ever so regular; that alone, without some new argument or inference, proves not that, for the future, it will continue so. In vain do you pretend to have learned the nature of bodies from your past experience. Their secret nature, and consequently all their effects and influence, may change, without any change in their sensible qualities. This happens sometimes, and with regard to some objects: Why may it not happen always, and with regard to all objects? What logic, what process of argument secures you against this supposition? My practice, you say, refutes my doubts. But you mistake the purport of my question. As an agent, I am quite satisfied in the point; but as a philosopher, who has some share of curiosity, I will not say scepticism, I want to learn the foundation of this inference. No reading, no enquiry has yet been able to remove my difficulty, or give me satisfaction in a matter of such importance. Can I do better than propose the difficulty to the public, even though, perhaps, I have small hopes of obtaining a solution? We shall at least, by this means, be sensible of our ignorance, if we do not augment our knowledge.

32. Someone might say that, from repeated uniform experiments, we infer a connection between observable qualities and hidden powers. But I must confess that this seems to be the same difficulty expressed in different words. The question remains: What reasoning is this inference based on? What are the intermediate ideas that connect two propositions so far apart? The color, texture, and other observable qualities of bread do not, by themselves, seem connected to its hidden powers to nourish and sustain us. This is generally admitted. Otherwise, we could infer those hidden powers from the first appearance of bread’s observable qualities, without experience. That would contradict the views of all philosophers and the plain facts. This, then, is our natural state of ignorance about the powers and effects of all objects. How does experience remedy it? Experience only shows us a number of uniform effects resulting from certain objects. It teaches us that those particular objects, at that particular time, had such powers and forces. When a new object appears with similar observable qualities, we expect similar powers and forces and look for a similar effect. From something with the same color and texture as bread, we expect the same nourishment and support. But this is surely a step the mind takes that needs explaining. Suppose someone says, In all past cases, I have found such observable qualities joined with such hidden powers. Then suppose he says, Similar observable qualities will always be joined with similar hidden powers. He is not merely repeating himself. The two propositions are not the same in any respect. You say that the second proposition is an inference from the first. But you must admit that the inference is not intuitive. Nor can it be proved by demonstration. What kind of inference is it, then? To say that the inference is based on experience is to assume the very point in question. For all inferences from experience assume, as their foundation, that the future will resemble the past. They also assume that similar powers will be joined with similar observable qualities. If there is any suspicion that the course of nature may change, and that the past may be no guide to the future, all experience becomes useless. It can give rise to no inference or conclusion. Therefore, no arguments from experience can prove that the past resembles the future. All such arguments are based on the assumption that it does. However regular the course of things has been up to now, that alone does not prove it will continue to be regular. Some new argument or inference would be needed. You claim in vain to have learned the nature of bodies from past experience. Their hidden nature, and therefore all their effects and influence, may change without any change in their observable qualities. This sometimes happens with some objects. Why might it not always happen with all objects? What logic, what reasoning, protects you against this possibility? You say that my actions refute my doubts. But you misunderstand the purpose of my question. In my actions, I am quite satisfied on this point. But as a philosopher with some curiosity, if not skepticism, I want to learn the basis of this inference. No reading or inquiry has yet removed my difficulty or satisfied me on a matter of such importance. Can I do better than put the difficulty before the public, even though I may have little hope of obtaining a solution? At least, by doing so, we shall become aware of our ignorance, even if we do not increase our knowledge.

33. I must confess that a man is guilty of unpardonable arrogance who concludes, because an argument has escaped his own investigation, that therefore it does not really exist. I must also confess that, though all the learned, for several ages, should have employed themselves in fruitless search upon any subject, it may still, perhaps, be rash to conclude positively that the subject must, therefore, pass all human comprehension. Even though we examine all the sources of our knowledge, and conclude them unfit for such a subject, there may still remain a suspicion, that the enumeration is not complete, or the examination not accurate. But with regard to the present subject, there are some considerations which seem to remove all this accusation of arrogance or suspicion of mistake.

33. I must admit that a person is inexcusably arrogant to conclude that an argument does not exist because they have failed to find it. I must also admit that scholars might spend centuries searching for answers on a subject without success. Even then, it may perhaps be rash to conclude with certainty that the subject is beyond all human understanding. We might examine all the sources of our knowledge and conclude that none is suited to the subject. Even so, we may still suspect that we have not listed every source or examined them accurately. But in the present case, some considerations seem to remove any grounds for accusing me of arrogance or suspecting an error.

It is certain that the most ignorant and stupid peasants—nay infants, nay even brute beasts—improve by experience, and learn the qualities of natural objects, by observing the effects which result from them. When a child has felt the sensation of pain from touching the flame of a candle, he will be careful not to put his hand near any candle; but will expect a similar effect from a cause which is similar in its sensible qualities and appearance. If you assert, therefore, that the understanding of the child is led into this conclusion by any process of argument or ratiocination, I may justly require you to produce that argument; nor have you any pretence to refuse so equitable a demand. You cannot say that the argument is abstruse, and may possibly escape your enquiry; since you confess that it is obvious to the capacity of a mere infant. If you hesitate, therefore, a moment, or if, after reflection, you produce any intricate or profound argument, you, in a manner, give up the question, and confess that it is not reasoning which engages us to suppose the past resembling the future, and to expect similar effects from causes which are, to appearance, similar. This is the proposition which I intended to enforce in the present section. If I be right, I pretend not to have made any mighty discovery. And if I be wrong, I must acknowledge myself to be indeed a very backward scholar; since I cannot now discover an argument which, it seems, was perfectly familiar to me long before I was out of my cradle.

It is certain that even the most ignorant and stupid peasants, and even infants and animals, learn from experience. They learn the qualities of natural objects by observing their effects. When a child feels pain from touching a candle flame, he will be careful not to put his hand near any candle. He will expect a similar effect from a cause that looks and feels similar. Suppose, then, you claim that the child reaches this conclusion through some process of reasoning. I can fairly ask you to produce the argument, and you have no excuse for refusing such a reasonable request. You cannot say that the argument is difficult and might escape your investigation. After all, you admit that it is obvious enough for a mere infant to grasp. So if you hesitate for a moment, you more or less concede the point. You also concede it if, after reflection, you produce a complex or profound argument. In either case, you admit that reasoning does not lead us to suppose that the future will resemble the past. Nor does reasoning lead us to expect similar effects from causes that appear similar. This is the point I intended to establish in this section. If I am right, I do not claim to have made any great discovery. And if I am wrong, I must admit that I am indeed a very slow learner. For I cannot now discover an argument that, it seems, I knew perfectly well long before I left my cradle.