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 include only the possibility of existence, necessary existence is inseparable from the concept of the most perfect being.  God cannot be thought apart from existence; he has the ground of his existence in himself; he is a se or causa sui.  Finally, Descartes adds a third argument.  The idea of perfections which I do not possess can only have been imparted to me by a more perfect being than I, which has bestowed on me all that I am and all that I am capable of becoming.  If I had created myself, I would have bestowed upon myself these absent perfections also.  And the existence of a plurality of causes is negatived by the supreme perfection which I conceive in the idea of God, the indivisible unity of his attributes.  Among the attributes of God his veracity is of special importance.  It is impossible that he should will to deceive us; that he should be the cause of our errors.  God would be a deceiver, if he had endowed us with a reason to which error should appear true, even when it uses all its foresight in avoiding it and assents only to that which it clearly and distinctly perceives.  Error is man’s own fault; he falls into it only when he misuses the divine gift of knowledge, which includes its own standard.  Thus Descartes finds new confirmation for his test of truth in the veracitas dei.  Erdmann has given a better defense of Descartes than the philosopher himself against the charge that this is arguing in a circle, inasmuch as the existence of God is proved by the criterion of truth, and then the latter by the former:  The criterion of certitude is the ratio cognoscendi of God’s existence; God is the ratio essendi of the criterion of certitude.  In the order of existence God is first, he creates the reason together with its criterion; in the order of knowledge the criterion precedes, and God’s existence follows from it.  Descartes himself endeavors to avoid the circle by making intuitive knowledge self-evident, and by not bringing in the appeal to God’s veracity in demonstrative knowledge until, in reflective thought, we no longer have each separate link in the chain of proof present to our minds with full intuitive certainty, but only remember that we have previously understood the matter with clearness and distinctness.