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.
