Now that Descartes has shown that God exists, he has only to show that God is the cause of our clear and distinct perceptions and that God is not a deceiver, and we will be able to use our clear and distinct perceptions in order to build up a systematic body of certain knowledge. This is precisely what Descartes does in principles 19–30. In the process of establishing these claims, however, Descartes also draws many other conclusions regarding the nature of God and our relation to him.
Descartes gives several proofs for the claim that we (and thus our faculty of reason, responsible for these clear and distinct perceptions) were created by God. The first of these comes in principle I.20. Like the causal argument for existence, this proof rests on the fact that we have an idea of a supremely perfect being. Since we have this idea, Descartes claims, it is clear that we cannot be the authors of our own being. If we were the authors of our own being, we would have given ourselves all of the perfections of which we can conceive. Clearly, though, we do not have all of these perfections. The next argument comes in principle I.21. Not only do we need some explanation for our initial entrance into existence, he tells us, but we also need to find some cause that explains why we continue to exist from moment to moment. There is nothing in the idea of existing at one time to imply existence at a later time. Certainly, if we ourselves were causally responsible for this feat we would be aware of that fact.
Next, he moves on to elaborate on the nature of God. Though we cannot know the full nature of God, we do know that God is absolutely perfect—this property is contained in our very idea of him. Using only this piece of knowledge regarding God's nature, Descartes is now in a position to dispel all doubts concerning clear and distinct perception, which he does in principle I.30: if God gave us a faculty which presented certain propositions as indubitably true, when in fact they were not, then God would be a deceiver. However, to be a deceiver implies being malicious, which is a defect, and God, being perfect, does not possess any defects. Therefore, Descartes can conclude, we can trust our clear and distinct perceptions to tell the truth.
Before drawing this important conclusion, however, Descartes takes the time to establish some other facts about God. First, Descartes points out, he is not corporeal, but mental, because corporeality involves imperfection. Next, he is careful to mention that we must believe everything that God has revealed to us (such as the Trinity) even if we do not understand it. Finally, he discusses the difference between the property of infiniteness, which is a positive conception, and indefiniteness, which is a negative conception. Among our ideas, only our idea of God includes the notion of the infinite. Only with God do we positively know that there is no limit to him. All of our other ideas, which include the property of limitlessness (e.g. our idea of the number of grains of sand in the world), are merely representing the indefinite. That is to say, in these cases, all that we perceive is that we cannot perceive a limit; we do not perceive that there is no limit.
The argument that Descartes gives in Principle I.19—for the claim that we (and thus our faculty of reason) are created by God—is surprisingly skimpy. In the Meditations he gives a much more robust version of the same argument. He sets the argument up by considering all the plausible candidates for the position of author of his existence. He comes up with three: God, himself, or some other being less perfect than God, such as his parents.
He rules himself out in much the same way as he rules himself out in the Principles. If he were the author of his own existence, he would have made himself far more perfect. In addition, he adds another count against himself: if he were the author of his own existence he would certainly have given himself this knowledge. In other words, if he were the author of his own existence, he would know that he was the author of his own existence. Finally, what is even more certain is that if he were responsible for keeping himself in existence from moment to moment, he would know about this feat.
Next Descartes turns to the possibility that the author of his existence is some being lesser than God, such as his parents. This, however, he rules out on the grounds that no being lesser than God could have created the idea of God that is in him. Since he has this idea, some being of infinite reality must have put it inside of him, and this being of infinite reality must thus be his creator. Descartes, in fact, argues that our idea of God, which is an innate idea, was placed in us by God as the artist's signature on his handiwork.
Before concluding that God is the author of his being, though, he considers one last possibility. Perhaps what caused this idea of infinite perfection in us is not a single being, but a whole collection of causes. In other words, perhaps we got the ideas of different perfections (e.g goodness, truth, eternality) from different sources. Descartes rules out this possibility on the grounds that unity, or the "inseparability of all the attributes of God," is one of the key components of our idea of God. Descartes is eager to point out that this extended argument for God as our creator, can double as yet a third argument for God's existence. If God must be posited in order to account for our existence, then God himself must exist.
One last issue that deserves attention from this section of the Principles is Descartes' discussion of the difference between the infinite and the indefinite. This conceptual analysis, which takes place at I.27, might sound like it is beside the point of the project at hand, but it is actually extremely important. It is intended as further proof that our idea of God can only be caused by God himself. As far as Descartes is concerned, there are only three possible ways that we could have arrived at the notion of the infinite. The first possibility is that we might have taken the idea of finitude and negated it in order to obtain the idea of the infinite. However, this would give us a negative idea of infinity, not a positive idea; we would think of infinity as a lack of finitude, when, really, it is the other way around. Alternatively, we might have started with our idea of the finite and extrapolated, continually adding more and more, until we recognized that we could potentially add on like this forever. This, Descartes, claims, is how we get to the idea of the indefinite. This method of extrapolation lands us with a vague sense that addition need never end, but it does not afford us with a positive conception of unendingness. Finally, there is the third possibility: God placed this idea in us. Given that there seems to be no other way that we could have arrived at this notion, Descartes concludes that this last scenario is the correct one.