6. The truth of few universal propositions concerning substances is to be known.
On the other side, the names of
substances, when made use of as they should be, for the ideas men have in their minds, though they carry a clear
and determinate signification with them, will not yet serve us to make many universal propositions of whose truth
we can be certain. Not because in this use of them we are uncertain what things are signified by them, but because
the complex ideas they stand for are such combinations of simple ones as carry not with them any discoverable
connexion or repugnancy, but with a very few other ideas.
