12. So far as our complex ideas agree with those archetypes without us, so far our knowledge concerning
substances is real.
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 inconsistence, 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 inconsistence
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 further than experience and sensible observation reach. 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 shown) 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 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.
