— 582 —
Because Co-existence of Ideas in few Cases to be known.
      §7. The complex Ideas, that our Names of the Species of Sub-
stances 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.
3. 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 farther, 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
Locke Hum IV, 6, §7, p. 582