from saying
or intending they should be laid aside; as some have been too forward
to charge me. I affirm them to be truths, self-evident truths; and so
cannot be laid aside. As far as their influence will reach, it is in
vain to endeavour, nor will I attempt, to abridge it. But yet, without
any injury to truth or knowledge, I may have reason to think their use
is not answerable to the great stress which seems to be laid on them;
and I may warn men not to make an ill use of them, for the confirming
themselves in errors.]

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]

16. Instance in demonstratio