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 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