THE IDEA OF SOLIDITY

1. We receive this Idea from Touch.

1. We get this idea through touch.

The idea of SOLIDITY we receive by our touch: and it arises from the resistance which we find in body to the entrance of any other body into the place it possesses, till it has left it. There is no idea which we receive more constantly from sensation than solidity. Whether we move or rest, in what posture soever we are, we always feel something under us that supports us, and hinders our further sinking downwards; and the bodies which we daily handle make us perceive that, whilst they remain between them, they do, by an insurmountable force, hinder the approach of the parts of our hands that press them. THAT WHICH THUS HINDERS THE APPROACH OF TWO BODIES, WHEN THEY ARE MOVED ONE TOWARDS ANOTHER, I CALL SOLIDITY. I will not dispute whether this acceptation of the word solid be nearer to its original signification than that which mathematicians use it in. It suffices that I think the common notion of solidity will allow, if not justify, this use of it; but if any one think it better to call it IMPENETRABILITY, he has my consent. Only I have thought the term solidity the more proper to express this idea, not only because of its vulgar use in that sense, but also because it carries something more of positive in it than impenetrability; which is negative, and is perhaps more a consequence of solidity, than solidity itself. This, of all other, seems the idea most intimately connected with, and essential to body; so as nowhere else to be found or imagined, but only in matter. And though our senses take no notice of it, but in masses of matter, of a bulk sufficient to cause a sensation in us: yet the mind, having once got this idea from such grosser sensible bodies, traces it further, and considers it, as well as figure, in the minutest particle of matter that can exist; and finds it inseparably inherent in body, wherever or however modified.

We get the idea of SOLIDITY through touch. This idea comes from the resistance a body offers when another body tries to enter the space it occupies. The first body resists until it leaves that space. No idea comes to us more constantly through sensation than solidity. Whether we move or rest, whatever our posture, we always feel something beneath us that supports us. This support prevents us from sinking farther down. The objects we handle every day also make us aware of solidity. As long as these objects remain between the parts of our hands pressing against them, they keep those parts apart. They do so with a force we cannot overcome. I CALL SOLIDITY THE PROPERTY THAT KEEPS TWO BODIES APART WHEN THEY ARE MOVED TOWARD EACH OTHER. I will not debate whether this meaning of solid is closer to its original meaning than the mathematicians’ meaning. It is enough that I think the common understanding of solidity allows, if not justifies, my use of the word. But if anyone prefers to call it IMPENETRABILITY, I agree. Still, I have thought solidity the better term for this idea, partly because people commonly use it in this sense. Solidity also expresses something more positive than impenetrability does. Impenetrability expresses a negative idea and is perhaps a consequence of solidity rather than solidity itself. Of all ideas, solidity seems the most closely connected with bodies and the most essential to them. Solidity can be found or imagined nowhere except in matter. Our senses detect solidity only in masses of matter large enough to produce a sensation in us. Yet once the mind gains this idea from larger, perceptible bodies, it extends the idea further. The mind considers solidity, like shape, to exist in the smallest particle of matter that can exist. It finds solidity inseparable from every body, wherever that body is and whatever form it takes.

2. Solidity fills Space.

2. Solidity fills space.

This is the idea which belongs to body, whereby we conceive it to fill space. The idea of which filling of space is,—that where we imagine any space taken up by a solid substance, we conceive it so to possess it, that it excludes all other solid substances; and will for ever hinder any other two bodies, that move towards one another in a straight line, from coming to touch one another, unless it removes from between them in a line not parallel to that which they move in. This idea of it, the bodies which we ordinarily handle sufficiently furnish us with.

This is the idea we associate with a body when we think of it as filling space. By filling space, we mean that a solid substance occupies space in a way that excludes all other solid substances. A solid substance between two other bodies moving toward each other in a straight line will always prevent them from touching. They can touch only if the substance between them moves away along a line not parallel to their movement. The bodies we handle in everyday life give us ample experience of this idea.

3. Distinct from Space.

3. Distinct from space.

This resistance, whereby it keeps other bodies out of the space which it possesses, is so great, that no force, how great soever, can surmount it. All the bodies in the world, pressing a drop of water on all sides, will never be able to overcome the resistance which it will make, soft as it is, to their approaching one another, till it be removed out of their way: whereby our idea of solidity is distinguished both from pure space, which is capable neither of resistance nor motion; and from the ordinary idea of hardness. For a man may conceive two bodies at a distance, so as they may approach one another, without touching or displacing any solid thing, till their superficies come to meet; whereby, I think, we have the clear idea of space without solidity. For (not to go so far as annihilation of any particular body) I ask, whether a man cannot have the idea of the motion of one single body alone, without any other succeeding immediately into its place? I think it is evident he can: the idea of motion in one body no more including the idea of motion in another, than the idea of a square figure in one body includes the idea of a square figure in another. I do not ask, whether bodies do so EXIST, that the motion of one body cannot really be without the motion of another. To determine this either way, is to beg the question for or against a VACUUM. But my question is,—whether one cannot have the IDEA of one body moved, whilst others are at rest? And I think this no one will deny. If so, then the place it deserted gives us the idea of pure space without solidity; whereinto any other body may enter, without either resistance or protrusion of anything. When the sucker in a pump is drawn, the space it filled in the tube is certainly the same whether any other body follows the motion of the sucker or not: nor does it imply a contradiction that, upon the motion of one body, another that is only contiguous to it should not follow it. The necessity of such a motion is built only on the supposition that the world is full; but not on the distinct IDEAS of space and solidity, which are as different as resistance and not resistance, protrusion and not protrusion. And that men have ideas of space without a body, their very disputes about a vacuum plainly demonstrate, as is shown in another place.

A body resists other bodies entering the space it occupies. This resistance is so great that no force, however great, can overcome it. All the bodies in the world, pressing a drop of water from every side, could never overcome its resistance. Soft as the drop is, it will prevent those bodies from coming together until it is moved out of their way. This distinguishes our idea of solidity from both pure space and the ordinary idea of hardness. Pure space can neither resist nor move. For we can imagine two bodies some distance apart, moving toward each other without touching or displacing anything solid. They can continue until their surfaces meet. This, I think, gives us a clear idea of space without solidity. We need not go so far as to consider the annihilation of any particular body. I ask only whether we can imagine a single body moving without another immediately taking its place. I think we clearly can. The idea of one body moving does not include the idea of another moving. In the same way, the idea of one square body does not include the idea of another square body. I am not asking whether bodies actually EXIST in such a way that one cannot move without another moving. To decide either way would be to assume the answer to the question of whether a VACUUM exists. My question is whether we can have the IDEA of one body moving while others remain at rest. And I think no one will deny that we can. If so, the place the body leaves gives us the idea of pure space without solidity. Any other body could enter that space without meeting resistance or pushing anything out of the way. When a pump’s piston is drawn back, the space it occupied in the tube certainly remains the same. This is true whether another body follows the piston’s movement or not. Nor is there any contradiction in imagining one body moving without another body, merely touching it, following along. The supposed necessity of that following movement rests only on the assumption that the world is completely filled with bodies. It does not rest on the distinct IDEAS of space and solidity. Those ideas differ as much as resistance differs from no resistance, or pushing something aside differs from not doing so. And people’s very disputes about a vacuum plainly show that they have ideas of space without a body. This is shown elsewhere.

4. From Hardness.

4. From hardness.

Solidity is hereby also differenced from hardness, in that solidity consists in repletion, and so an utter exclusion of other bodies out of the space it possesses: but hardness, in a firm cohesion of the parts of matter, making up masses of a sensible bulk, so that the whole does not easily change its figure. And indeed, hard and soft are names that we give to things only in relation to the constitutions of our own bodies; that being generally called hard by us, which will put us to pain sooner than change figure by the pressure of any part of our bodies; and that, on the contrary, soft, which changes the situation of its parts upon an easy and unpainful touch.

Solidity also differs from hardness in this respect. Solidity consists in filling space and completely excluding other bodies from the space a body occupies. But hardness consists in the firm binding of matter’s parts into masses large enough for us to perceive. These parts hold together so that the whole mass does not easily change shape. Indeed, we call things hard or soft only in relation to the nature of our own bodies. We generally call something hard if pressing any part of our bodies against it causes pain before changing its shape. By contrast, we call something soft if a gentle, painless touch changes the arrangement of its parts.

But this difficulty of changing the situation of the sensible parts amongst themselves, or of the figure of the whole, gives no more solidity to the hardest body in the world than to the softest; nor is an adamant one jot more solid than water. For, though the two flat sides of two pieces of marble will more easily approach each other, between which there is nothing but water or air, than if there be a diamond between them; yet it is not that the parts of the diamond are more solid than those of water, or resist more; but because the parts of water, being more easily separable from each other, they will, by a side motion, be more easily removed, and give way to the approach of the two pieces of marble. But if they could be kept from making place by that side motion, they would eternally hinder the approach of these two pieces of marble, as much as the diamond; and it would be as impossible by any force to surmount their resistance, as to surmount the resistance of the parts of a diamond. The softest body in the world will as invincibly resist the coming together of any other two bodies, if it be not put out of the way, but remain between them, as the hardest that can be found or imagined. He that shall fill a yielding soft body well with air or water, will quickly find its resistance. And he that thinks that nothing but bodies that are hard can keep his hands from approaching one another, may be pleased to make a trial, with the air inclosed in a football. The experiment, I have been told, was made at Florence, with a hollow globe of gold filled with water, and exactly closed; which further shows the solidity of so soft a body as water. For the golden globe thus filled, being put into a press, which was driven by the extreme force of screws, the water made itself way through the pores of that very close metal, and finding no room for a nearer approach of its particles within, got to the outside, where it rose like a dew, and so fell in drops, before the sides of the globe could be made to yield to the violent compression of the engine that squeezed it.

But difficulty in rearranging a body's perceptible parts, or changing its overall shape, does not make that body more solid. The hardest body in the world is no more solid than the softest. A diamond is not the slightest bit more solid than water. The flat faces of two pieces of marble can approach each other more easily with water or air between them. A diamond between the marble pieces makes their approach harder. But this is not because the diamond's parts are more solid than water's parts, or offer greater resistance. Rather, water's parts separate more easily from one another. They can move sideways more easily, making way for the two pieces of marble to approach. But suppose the water's parts could be prevented from moving sideways to make room. They would forever prevent the marble pieces from approaching, just as a diamond would. No force could overcome their resistance, any more than it could overcome the resistance of a diamond's parts. The softest body in the world can resist two other bodies coming together just as completely as the hardest imaginable. This holds if the soft body remains between the other two bodies and is not pushed out of the way. Anyone who thoroughly fills a soft, flexible body with air or water will quickly discover its resistance. Anyone who thinks only hard bodies can keep their hands from coming together may try squeezing air enclosed in a football. I have been told that an experiment was made at Florence using a hollow gold sphere. The sphere was filled with water and tightly sealed. This experiment further demonstrates the solidity of something as soft as water. The filled gold sphere was put into a press driven by the immense force of screws. The water forced its way through the pores of that very dense metal. Because the water's particles had no room to move closer together inside, the water escaped to the outside. There the water appeared like dew and then fell in drops. This happened before the sphere's sides could be made to give way under the press's violent compression.

5. On Solidity depend Impulse, Resistance and Protrusion.

5. Pushing, resistance, and forcing objects out of place depend on solidity.

By this idea of solidity is the extension of body distinguished from the extension of space:—the extension of body being nothing but the cohesion or continuity of solid, separable, movable parts; and the extension of space, the continuity of unsolid, inseparable, and immovable parts. Upon the solidity of bodies also depend their mutual impulse, resistance, and protrusion. Of pure space then, and solidity, there are several (amongst which I confess myself one) who persuade themselves they have clear and distinct ideas; and that they can think on space, without anything in it that resists or is protruded by body. This is the idea of pure space, which they think they have as clear as any idea they can have of the extension of body: the idea of the distance between the opposite parts of a concave superficies being equally as clear without as with the idea of any solid parts between: and on the other side, they persuade themselves that they have, distinct from that of pure space, the idea of SOMETHING THAT FILLS SPACE, that can be protruded by the impulse of other bodies, or resist their motion. If there be others that have not these two ideas distinct, but confound them, and make but one of them, I know not how men, who have the same idea under different names, or different ideas under the same name, can in that case talk with one another; any more than a man who, not being blind or deaf, has distinct ideas of the colour of scarlet and the sound of a trumpet, could discourse concerning scarlet colour with the blind man I mentioned in another place, who fancied that the idea of scarlet was like the sound of a trumpet.

The idea of solidity distinguishes the extension of a body from the extension of space. A body’s extension is simply the joining or continuity of solid parts that can be separated and moved. Space’s extension is the continuity of parts that are not solid and cannot be separated or moved. Solidity also allows bodies to push one another, resist one another, and displace one another. Several people believe they have clear and distinct ideas of both pure space and solidity. I admit that I am one of them. They believe they can think of space containing nothing that resists a body or is displaced by one. They think their idea of pure space is as clear as any idea they can have of a body’s extension. Consider the distance between opposite parts of a hollow surface. The idea of that distance is equally clear whether or not we imagine solid parts between them. On the other hand, these people believe they have another idea, distinct from their idea of pure space. This is the idea of SOMETHING THAT FILLS SPACE and can be displaced when other bodies push against it. It can also resist their motion. Others may not distinguish these two ideas, but confuse them and treat them as one. If so, I do not know how people can discuss the subject with one another. They may use different names for the same idea, or the same name for different ideas. Consider someone who is neither blind nor deaf, with distinct ideas of scarlet and the sound of a trumpet. How could that person discuss scarlet with the blind man I mentioned elsewhere? That blind man imagined that the idea of scarlet was like the sound of a trumpet.

6. What Solidity is.

6. What solidity is.

If any one asks me, WHAT THIS SOLIDITY IS, I send him to his senses to inform him. Let him put a flint or a football between his hands, and then endeavour to join them, and he will know. If he thinks this not a sufficient explication of solidity, what it is, and wherein it consists; I promise to tell him what it is, and wherein it consists, when he tells me what thinking is, or wherein it consists; or explains to me what extension or motion is, which perhaps seems much easier. The simple ideas we have, are such as experience teaches them us; but if, beyond that, we endeavour by words to make them clearer in the mind, we shall succeed no better than if we went about to clear up the darkness of a blind man’s mind by talking; and to discourse into him the ideas of light and colours. The reason of this I shall show in another place.

If anyone asks me WHAT SOLIDITY IS, I direct him to his senses for the answer. Let him put a flint or a football between his hands, then try to bring his hands together. Then he will know. If he thinks this does not adequately explain what solidity is or what it consists in, I promise to explain. But first, he must tell me what thinking is or what it consists in. Or he must explain what extension or motion is, a task that perhaps seems much easier. Our simple ideas are just what experience teaches us. But if we try to make these ideas clearer through words alone, we will do no better than this: trying, through talk, to dispel the darkness in a blind man’s mind and give him ideas of light and colours. I will explain the reason for this elsewhere.