For if the pressure of the aether be the adequate cause of
cohesion, wherever that cause operates not, there can be no
cohesion. And since it cannot operate against a lateral separation,
(as has been shown), therefore in every imaginary plane,
intersecting any mass of matter, there could be no more cohesion
than of two polished surfaces, which will always, notwithstanding
any imaginable pressure of a fluid, easily slide one from another.
So that perhaps, how clear an idea soever we think we have of the
extension of body, which is nothing but the cohesion of solid parts,
he that shall well consider it in his mind, may have reason to
conclude, That it is as easy for him to have a clear idea how the soul
thinks as how body is extended. For, since body is no further, nor
otherwise, extended, than by the union and cohesion of its solid
parts, we shall very ill comprehend the extension of body, without
understanding wherein consists the union and cohesion of its parts;
which seems to me as incomprehensible as the manner of thinking, and
how it is performed.
25. We can as little understand how the parts cohere in extension,
as how our spirits perceive or move. I allow it is usual for most
people to wonder how any one should find a difficulty in what they
think they every day observe. Do we not see (will they be ready to
say) the parts of bodies stick firmly together? Is there anything more
common? And what doubt can there be made of it? And the like, I say,
concerning thinking and voluntary motion.
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