— 606 —
Their Application dangerous about complex Ideas.
      §15. 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 farther than grounded on Experience. And though the Conse-
quence 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, Vertue; there they are of infinite danger, and most
commonly make Men receive and retain Falshood for manifest
Truth, and Uncertainty for Demonstration: upon which follows
Errour, 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; as is clear
in the Demonstrations above-mentioned about a Vacuum. So that
whilst Men take Words for Things, as usually they do, these
Maximes may and do commonly serve to prove contradictory
Locke Hum IV, 7, §15, p. 606