Finally, we frequently observe that
heat [hardness, weight], and the other sensible qualities, as far as
they are in objects, and also the forms of those bodies that are
purely material, as, for example, the forms of fire, are produced in
them by the motion of certain other bodies, and that these in their
turn likewise produce other motions in other bodies. And we can
easily conceive how the motion of one body may be caused by that of
another, and diversified by the size, figure, and situation of its
parts, but we are wholly unable to conceive how these same things
(viz., size, figure, and motion), can produce something else of a
nature entirely different from themselves, as, for example, those
substantial forms and real qualities which many philosophers suppose
to be in bodies; nor likewise can we conceive how these qualities or
forms possess force to cause motions in other bodies. But since we
know, from the nature of our soul, that the diverse motions of body
are sufficient to produce in it all the sensations which it has, and
since we learn from experience that several of its sensations are in
reality caused by such motions, while we do not discover that
anything besides these motions ever passes from the organs of the
external senses to the brain, we have reason to conclude that we in
no way likewise apprehend that in external objects, which we call
light, colour, smell, taste, sound, heat or cold, and the other
tactile qualities, or that which we call their substantial forms,
unless as the various dispositions of these objects which have the
power of moving our nerves in various ways.
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