mathematics, whose trustworthiness he never seriously questioned, but only
hypothetically, in order to exhibit the still higher certainty of the "I
think, therefore I am." He wants to give philosophy the stable character
which had so impressed him in mathematics when he was a boy, and recommends
her, therefore, not merely the evidence of mathematics as a general
example, but the mathematical method for definite imitation. Metaphysics,
like mathematics, must derive its conclusions by deduction from
self-evident principles. Thus the geometrical method begins its rule in
philosophy, a rule not always attended with beneficial results.

With this criterion of truth Descartes advances to the consideration of
ideas. He distinguishes volition and judgment from ideas in the narrow
sense (_imagines_), and divides the latter, according to their origin, into
three classes: _ideae innatae, adventitiae, a me ipso factae_, considering
the second class, the "adventitious" ideas, the most numerous, but the
first, the "innate" ideas, the most important. No idea is higher or clearer
than the idea of God or the most perfect being. Whence comes this idea?
That every idea must have a cause, follows from the "clear and distinct"
principle that nothing produces nothing. It follows from this same
principle, _ex nihilo nihil fit_, however, that the cause must contain as
much reality or perfection--_realitas_ and _perfectio_ are synonymous--as
the effect, for otherwise the overplus would have come from nothing. So
much ("objective," representative) reality contained in an idea, so much or
more ("formal," actual) reality must be contained in its cause. The idea
of God as infinite, independent, omnipotent, omniscient, and creative
substance, has not come to me through the senses, nor have I formed it
myself. The power to conceive a being more perfect than myself, can have
only come from someone who is more perfect in reality than I. Since I know
that the infinite contains more reality than the finite, I may conclude
that the idea of the infinite has not been derived from the idea of the