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
finite by abstraction and negation; it precedes the latter, and I become
conscious of my defects and my finitude only by comparison with the
absolute perfection of God. This idea, then, must have been implanted in me
by God himself. The idea of God is an original endowment; it is as innate
as the idea of myself. However incomplete it may be, it is still
sufficient to give a knowledge of God's existence, although not a perfect
comprehension of his being, just as a man may skirt a mountain without
encircling it.

Descartes brings in the idea of God in order to escape solipsism. So long
as the self-consciousness of the ego remained the only certainty, there was
no conclusive basis for the assumption that anything exists beyond self,
that the ideas which apparently come from without are really occasioned by
external things and do not spring from the mind itself. For our natural
instinct to refer them to objects without us might well be deceptive. It is
only through the idea of God, and by help of the principle that the cause
must contain at least as much reality as the effect, that I am taken beyond
myself and assured that I am not the only thing in the world. For as this
idea contains more of representative, than I of actual reality, I cannot
have been its cause.

To this empirical argument, which derives God's existence from our idea
of God (from the fact that we have an idea of him), Descartes joins the
(modified) ontological argument of Anselm, which deduces the existence of
God from the concept of God. While the ideas of all other things incl