Front Page Titles (by Subject) CHAP. IV.: Of Solidity. - The Works, vol. 1 An Essay concerning Human Understanding Part 1
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CHAP. IV.: Of Solidity. - John Locke, The Works, vol. 1 An Essay concerning Human Understanding Part 1 
The Works of John Locke in Nine Volumes, (London: Rivington, 1824 12th ed.). Vol. 1.
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We receive this idea from touch.
§ 1. 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 farther 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 no-where 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 farther; 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.
Solidity fills space.
§ 2. 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.
Distinct from space.
§ 3. 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 any thing. 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 showed in another place.
§ 4. 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.
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 foot-ball. The experiment, I have been told, was made at Florence, with a hollow globe of gold filled with water and exactly closed, which farther 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.
On solidity depend impulse, resistance, and protrusion.
§ 5. 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, moveable parts; and the extension of space, the continuity of unsolid, inseparable, and immoveable 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 any thing 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 mention in another place, who fancied that the idea of scarlet was like the sound of a trumpet.
What it is.
§ 6. If any one ask me, what this solidity is? I send him to his senses to inform him: let him put a flint or a foot-ball 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.