14. What is requisite for our knowledge of substances. Before we can
have any tolerable knowledge of this kind, we must First know what
changes the primary qualities of one body do regularly produce in
the primary qualities of another, and how. Secondly, We must know what
primary qualities of any body produce certain sensations or ideas in
us. This is in truth no less than to know all the effects of matter,
under its divers modifications of bulk, figure, cohesion of parts,
motion and rest. Which, I think every body will allow, is utterly
impossible to be known by us without revelation. Nor if it were
revealed to us what sort of figure, bulk, and motion of corpuscles
would produce in us the sensation of a yellow colour, and what sort of
figure, bulk, and texture of parts in the superficies of any body were
fit to give such corpuscles their due motion to produce that colour;
would that be enough to make universal propositions with certainty,
concerning the several sorts of them; unless we had faculties acute
enough to perceive the precise bulk, figure, texture, and motion of
bodies, in those minute parts, by which they operate on our senses, so
that we might by those frame our abstract ideas of them. I have
mentioned here only corporeal substances, whose operations seem to lie
more level to our understandings.
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