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Of Solidity

§ 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 moving 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 justifie, 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 it self. This of all other, seems the Idea most intimately connected with, and essential to Body, so as no where else to be found or imagin’d, but only [116]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, where-ever or however modified.

§ 2. This is the Idea 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 two other Bodies, that move towards one another in a strait 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.

§ 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, as 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. […]

§ 4. Solidity is hereby also differenced from Hardness, in that Solidity consists in repletion, and so an utter Exclusion of [118]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 call’d 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 easie, and unpainful touch. […]

§ 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, movable Parts; and the Extension of Space, the continuity of unsolid, inseparable, and immoveable Parts. Upon the Solidity of Bodies also depends their mutual Impulse, Resistance, and Protrusion. […]

§ 6. 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 Foot-ball between his Hands; and then endeavour to join them, and he will know. […] 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 shew, in another Place.