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So far as they agree with those, so far our Knowledge concerning them in real.
      §12. I say then, that to have Ideas of Substances, which, by being
conformable to Things, may afford us real Knowledge, it is not
enough, as in Modes, to put together such Ideas as have no in-
consistence, though they did never before so exist. V.g. the Ideas of
Sacrilege or Perjury, etc. were as real and true Ideas before, as after the
existence of any such fact. But our Ideas of Substances being supposed
Copies, and referred to Archetypes without us, must still be taken
from something that does or has existed; they must not consist of
Ideas put together at the pleasure of our Thoughts, without any
real pattern they were taken from, though we can perceive no in-
consistence in such a Combination. The reason whereof is, because
we knowing not what real Constitution it is of Substances, whereon
our simple Ideas depend, and which really is the cause of the strict
union of some of them one with another, and the exclusion of
others; there are very few of them, that we can be sure are, or are
not inconsistent in Nature, any farther than Experience and sensible
Observation reaches. Herein therefore is founded the reality of our
Knowledge concerning Substances, that all our complex Ideas of them
must be such, and such only, as are made up of such simple ones, as
have been discovered to co-exist in Nature. And our Ideas being thus
true, though not, perhaps, very exact Copies, are yet the Subjects of
real (as far as we have any) Knowledge of them. Which (as has been
already shewed) will not be found to reach very far: But so far as it
does, it will still be real Knowledge. Whatever Ideas we have, the
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Agreement we find they have with others, will still be knowledge.
If those Ideas be abstract, it will be general Knowledge. But to make
it real concerning Substances, the Ideas must be taken from the real
existence of things. Whatever simple Ideas have been found to co-
exist in any Substance, these we may with confidence join together
again, and so make abstract Ideas of Substances. For whatever have
once had an union in Nature, may be united again.
Locke Hum IV, 4, §12, pp. 568-569