5. On solidity depend impulse, resistance, and protrusion.
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.
