Now that we have some story about how our ideas of substances are constructed, we need to look at the two main kinds of substance we seem to find in the world: mind and body. Notice Locke’s argument against Descartes’s conflation of body and extension. Locke also replies here to Leibniz’s argument against Newtonian space, namely, that it must be either a substance or an accident, and neither makes much sense.
(From II.xiii.17—Cohesion of solid parts and Impulse, the primary ideas peculiar to Body) The primary ideas we have peculiarto body, as contradistinguished to spirit, are the cohesion of solid, and consequently separable, parts, and a power of communicating motion by impulse. These, I think, are the original ideas proper and peculiar to body; for figure is but the consequence of finite extension.
(From II.xiii.11—Extension and Body not the same) There are some that would persuade us, that body and extension are the same thing … If, therefore, they mean by body and extension the same
that other people do, viz. By body something that is solid and extended, whose parts are separable and movable different ways; and by extension, only the space that lies between the extremities of those solid coherent parts, and which is possessed by them, [then] they confound very different ideas one with
another; for I appeal to every man’s own thoughts, whether the idea of space be not as distinct from that of solidity, as it is from the idea of scarlet colour? It is true, solidity cannot
exist without extension, neither can scarlet colour exist without extension, but this hinders not, but that they are distinct ideas.
And if it be a reason to prove that spirit is different from body, because thinking includes not the idea of extension in it; the same reason will be as valid, I suppose, to prove that space is not body, because it includes not the idea of solidity in it; space and solidity being as distinct ideas as thinking and extension, and as wholly separable in the mind one from another … Extension includes no solidity, nor resistance to the motion of body, as body does.
(From II.xiii.3—Space and Extension) This space, considered barely in length between any two beings, without considering anything else between them, is called distance: if considered in length, breadth, and thickness, I think it may be called capacity. When considered between the extremities of matter, which fills the capacity of space with something solid, tangible, and moveable, it is properly called extension. And so extension is an idea belonging to body only; but space may, as is evident, be considered without it.
(From II.xiii.17—Substance, which we know not, no proof against space without body) If it be demanded (as usually it is) whether this space, void of body, be substance or accident, I shall readily answer I know not; nor shall be ashamed to own my ignorance, till they that ask show me a clear distinct idea of substance.