P34: Every idea that in us is absolute, or adequate and perfect, is true.

Dem.: When we say that there is in us an adequate and perfect idea, we are saying nothing but that (by P11 C) there is an adequate and perfect idea in God/Nature insofar as he constitutes the essence of our Mind, and consequently (by P32) we are saying nothing but that such an idea is true, q.e.d.