“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 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.