24. Not explained by an ambient fluid.
But, in truth, the pressure of any ambient fluid, how great soever, can be
no intelligible cause of the cohesion of the solid parts of matter. For, though such a pressure may hinder the
avulsion of two polished superficies, one from another, in a line perpendicular to them, as in the experiment of
two polished marbles; yet it can never in the least hinder the separation by a motion, in a line parallel to those
surfaces. Because the ambient fluid, having a full liberty to succeed in each point of space, deserted by a lateral
motion, resists such a motion of bodies, so joined, no more than it would resist the motion of that body were it on
all sides environed by that fluid, and touched no other body; and therefore, if there were no other cause of
cohesion, all parts of bodies must be easily separable by such a lateral sliding motion. 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.
