7. Because necessary co-existence of simple ideas in substances
can in few cases be known. The complex ideas that our names of the
species of substances properly stand for, are collections of such
qualities as have been observed to co-exist in an unknown
substratum, which we call substance; but what other qualities
necessarily co-exist with such combinations, we cannot certainly know,
unless we can discover their natural dependence; which, in their
primary qualities, we can go but a very little way in; and in all
their secondary qualities we can discover no connexion at all: for the
reasons mentioned, chap. iii. Viz. 1. Because we know not the real
constitutions of substances, on which each secondary quality
particularly depends. 2. Did we know that, it would serve us only
for experimental (not universal) knowledge; and reach with certainty
no further than that bare instance: because our understandings can
discover no conceivable connexion between any secondary quality and
any modification whatsoever of any of the primary ones. And
therefore there are very few general propositions to be made
concerning substances, which can carry with them undoubted certainty.
8. Instance in gold. "All gold is fixed," is a proposition whose
truth we cannot be certain of, how universally soever it be
believed.
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