15. They cannot add to our knowledge of substances, and their application to complex ideas is dangerous.
But let
them be of what use they will in verbal propositions, they cannot discover or prove to us the least knowledge of
the nature of substances, as they are found and exist without us, any further than grounded on experience. And
though the consequence of these two propositions, called principles, be very clear, and their use not dangerous or
hurtful, in the probation of such things wherein there is no need at all of them for proof, but such as are clear by
themselves without them, viz., where our ideas are [determined] and known by the names that stand for them: yet
when these principles, viz., what is, is, and it is impossible for the same thing to be and not to be, are made use of
in the probation of propositions wherein are words standing for complex ideas, v.g. man, horse, gold, virtue; there
they are of infinite danger, and most commonly make men receive and retain falsehood for manifest truth, and
uncertainty for demonstration: upon which follow error, obstinacy, and all the mischiefs that can happen from
wrong reasoning. The reason whereof is not, that these principles are less true or of less force in proving
propositions made of terms standing for complex ideas, than where the propositions are about simple ideas. But
because men mistake generally,--thinking that where the same terms are preserved, the propositions are about the
same things, though the ideas they stand for are in truth different, therefore these maxims are made use of to
support those which in sound and appearance are contradictory propositions; and is clear in the demonstrations
above mentioned about a vacuum. So that whilst men take words for things, as usually they do, these maxims may
and do commonly serve to prove contradictory propositions; as shall yet be further made manifest.
