Locke's ideas may be synopsized as follows: It is a sufficient account of solidity to say that it is got by touch and that it arises from the resistance found in bodies to the entrance of any other body. "It is that which hinders the approach of two bodies when they are moved towards one another." If not identical with matter, it is at all events its most essential property. "This of all others seems the idea most intimately connected with and essential to body, so as nowhere else to be found or imagined, but only in matter." It is, moreover, the source of the other properties of matter. "Upon the solidity of bodies depend their mutual impulse, resistance, and protrusion." Solidity, again, "is so inseparable an idea from body that upon that depends its filling of space, its contact, impulse, and communication of motion upon impulse." It is to be distinguished, therefore, from hardness, for hardness is relative and derived, various bodies having various degrees of it; while solidity consists in utter exclusion of other bodies from the space possessed by any one, so that the hardest body has no more solidity than the softest. The close connection between solidity and matter makes it not only possible, but necessary, to distinguish between matter and extension as against the Cartesians, who had identified them. In particular Locke notes three differences between these notions. Extension includes neither solidity nor resistance; its parts are inseparable from one another both really and mentally, and are immovable; while matter has solidity, its parts are mutually separable, and may be moved _in_ space. From this distinction between space and matter it follows, according to Locke, that there is such a thing as a vacuum, or that space is not necessarily a plenum of matter. Matter is that which fills space; but it is entirely indifferent to space whether or not it is filled. Space is occupied by matter, but there is no essential relation between them. Solidity is the essence of matter; emptiness is the characteristic of space. "The idea of space is as distinct from that of solidity as it is from that of scarlet color. It is true, solidity cannot exist without extension, neither can scarlet color exist without extension; but this hinders not that they are _distinct ideas_."