The next stage in the system, as outlined in the Meditations, seeks to establish that God exists. In his writings, Descartes made use of three principal arguments. The first (at least in the order of presentation in the Meditations) is a causal argument. While its fullest statement is in Meditation III, it is also found in the Discourse (Part IV) and in the Principles (Part I SSSS 17-18). The argument begins by examining the thoughts contained in the mind, distinguishing between the formal reality of an idea and its objective reality. The formal reality of any thing is just its actual existence and the degree of its perfection; the formal reality of an idea is thus its actual existence and degree of perfection as a mode of mind.

The objective reality of an idea is the degree of perfection it has, considered now with respect to its content. (This conception extends naturally to the formal and objective reality of a painting, a description or any other representation. ) In this connection, Descartes recognized three fundamental degrees of perfection connected with the capacity a thing has for independent existence, a hierarchy implicit in the argument of Meditation III and made explicit in the Third Replies (in response to Hobbes). The highest degree is that of an infinite substance (God), which depends on nothing; the next degree is that of a finite substance (an individual body or mind), which depends on God alone; the lowest is that of a mode (a property of a substance), which depends on the substance for its existence.

Descartes claims that 'it is manifest by the natural light that there must be at least as much reality in the efficient and total cause as in the effect of that cause'. From that he infers that there must be as much formal reality in the cause of an idea as there is objective reality in the idea itself. This is a bridge principle that allows Descartes to infer the existence of causes from the nature of the particular ideas that are in the mind, and thus are effects of some causes or another. In Meditation III, Descartes discusses various classes of ideas, one by one, and concludes that, as a finite substance, he can conceivably be the cause of all the ideas he has in his mind except for one: the idea of God. Since the idea of God is an idea of something that has infinite perfection, the only thing that can cause that idea in my mind is a thing that formally (actually) has the perfection that my idea has objectively - that is, God himself. Descartes used two other arguments for the existence of God in his writings.

In Meditation III, following the causal argument, he offers a version of the cosmological argument for those who, still blinded by the senses, may be reluctant to accept the bridge principle that his causal argument requires. (Versions of this argument are also found in Discourse Part IV, and in Principles Part I SSSS 20-1. ) This argument begins with the author's own existence, as established in Meditation II. But, the author might ask, what could have created me? It will not do, Descartes argues, to suggest that I have been in existence always, and thus I do not need a creator, since it takes as much power to sustain me from moment to moment as it does to create me anew.

I could not have created myself because then I would have been able to give myself all the perfections that I so evidently lack. Furthermore, if I could create myself, then I could also sustain myself, which I do not have the power to do; being a thinking thing, if I had such a power, I would be aware of having it. My parents cannot be my creators, properly speaking, since they have neither the ability to create a thinking thing (which is all I know myself to be at this stage of the Meditations), nor to sustain it once created. Finally, I could not have been created by another creature of lesser perfection than God, since I have an idea of God, an idea I could not acquire from a lesser being. (Here one suspects that this cosmological argument really collapses into the first causal argument. ) From this Descartes concludes 'that the mere fact that I exist and have within me an idea of a most perfect being...

provides a very clear proof that God indeed exists'. These first two arguments for the existence of God play a central role in the validation of reason, as discussed below. But after reason has been validated on theological grounds, Descartes presents in Meditation V a version of the ontological argument (see God, arguments for the existence of SSSS 2-3). After reflecting on the basis of geometric reasoning, the fact that 'everything which I clearly and distinctly perceive to belong to that thing really does belong to it', Descartes concludes that this applies to the idea of God as well.

Hence he concludes that 'it is quite evident that existence can no more be separated from the essence of God than the fact that its three angles equal to two right angles can be separated from the essence of a triangle, or than the idea of a mountain can be separated from the idea of a valley'. Though apparently circular in so far as its validity seems to depend on the prior arguments for the existence of God, it is not; Descartes' point is that 'even if it turned out that not everything on which I have meditated in these past days is true, I ought still to regard the existence of God as having at least the same level of certainty as I have hitherto attributed to the truths of mathematics'. As with the other two arguments, Descartes' ontological argument is also found in the Discourse (Part IV) and in the Principles (Part I SSSS 14-16); indeed, in the Principles it is the first argument he gives. As noted above, the existence of God plays a major role in the validation of reason. But it also plays a major role in two other parts of Descartes's ystem. As we shall later see in connection with Descartes' physics, God is the first cause of motion, and the sustainer of motion in the world.

Furthermore, because of the way he sustains motion, God constitutes the ground of the laws of motion. Finally, Descartes held that God is the creator of the so-called eternal truths. In a series of letters in 1630, Descartes enunciated the view that 'the mathematical truths which you call eternal have been laid down by God and depend on Him entirely no less than the rest of His creatures' (letter to Mersenne, 15 April 1630; Descartes 1984-91 vol 3: 23), a view that Descartes seems to have held into his mature years. While it never again gets the prominence it had in 1630, it is clearly present both in correspondence (for example, letter to Arnauld, 29 July 1648; Descartes 1984-91 vol 3: 358-9) and in published writings (for example, in the Sixth Responses).

Various commentators have proposed that Descartes was really an atheist, and that he includes the arguments for the existence of God as window dressing. While this is not impossible, the frequent appeal to God in philosophical contexts, both in private letters and in published work, suggests that it is rather unlikely.