P33: There is nothing positive in ideas on account of which they are called false.

Dem.: If you deny this, conceive (if possible) a positive mode of thinking which constitutes the form of error, or falsity. This mode of thinking cannot be in God/Nature (by P32). But it also can neither be nor be conceived outside God/Nature (by IP15). And so there can be nothing positive in ideas on account of which they are called false, q.e.d.