We are
so far from knowing what figure, size, or motion of parts produce a
yellow colour, a sweet taste, or a sharp sound, that we can by no
means conceive how any size, figure, or motion of any particles, can
possibly produce in us the idea of any colour, taste, or sound
whatsoever: there is no conceivable connexion between the one and
the other.
14. And seek in vain for certain and universal knowledge of
unperceived qualities in substances. In vain, therefore, shall we
endeavour to discover by our ideas (the only true way of certain and
universal knowledge) what other ideas are to be found constantly
joined with that of our complex idea of any substance: since we
neither know the real constitution of the minute parts on which
their qualities do depend; nor, did we know them, could we discover
any necessary connexion between them and any of the secondary
qualities: which is necessary to be done before we can certainly
know their necessary co-existence. So, that, let our complex idea of
any species of substances be what it will, we can hardly, from the
simple ideas contained in it, certainly determine the necessary
co-existence of any other quality whatsoever. Our knowledge in all
these inquiries reaches very little further than our experience.
Indeed some few of the primary qualities have a necessary dependence
and visible connexion one with another, as figure necessarily supposes
extension; receiving or communicating motion by impulse, supposes
solidity.
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