The crux of the argument is the surprising presence of the idea of the infinite God in the thought of finite man. Descartes argues that the idea of the infinite could not have originated in a finite being. Therefore we should seek the origin for more perfect ideas (finitude taken here as a lack of perfection) in a more perfect being, and the origin for the most perfect idea, infinity, in the most perfect being, God. Since we possess this most perfect idea, it is necessary that God exist as its origin.
Descartes summarizes as follows, “The whole force of the argument rests on the fact that I recognize that it would be impossible for me to exist, being of such a nature as I am (namely, having in me the idea of God), unless God did in fact exist.” (III, ln. 51-54)
How does Descartes reach this summary conclusion? In more detail, we can roughly retrace his significant steps as follows.
First, Descartes withdraws from the world of perception and the imagination of corporeal things. Where the relentless activity of thought makes it impossible to completely withdraw, he resolves to hold as false whatever bodies and images remain. In the previous meditations, Descartes has privileged the certainty of the mind over that of the body. If bodily things and their images are mutable and unsure, then we must turn to the barest activities of the mind to find that kernel of truth and certainty. The I is taken to be a thing that thinks (which is, to doubt, to affirm, to deny, to will, and more). Stripped to its most basic level, Descartes suggests that the I is in the company of what is true, as what is clearly and distinctly perceived.
Second, Descartes resubmits his criterion for truth – clear and distinct perceiving –to a skeptical critique. Is it possible to fall into error even in the company of what is clearly and distinctly perceived? If God wished such thing to be so, then even the most clear and distinct perceiving would be illusion. But is God a deceiver? Descartes forgoes all lesser forms of skepticism and formulates doubt in its most extreme and absolute form such that we finite beings, forged in illusion by the hand of an all powerful creator, would have no recourse by which we could measure the extent of our delusions.
Third, Descartes splits his thoughts into three general groups – ideas, volitions/affects, and judgments – and asks which could properly be deceived. Ideas and volitions/affects are rejected as susceptible to untruth; in judgments alone we make mistakes. We do not demand of ideas and volitions/affects that these thoughts correspond to things outside me. Only in judgments do we universally demand a truthful correlation between the thing we judge and the judgment.
Fourth, Descartes asks what we accept as justification for the truth of judgment, and distinguishes judgments “taught by nature” from judgments shown by the “light of nature.” It is the likenesses or unlikenesses of the ideas of our judgment to the things existing outside of us that persuades us to believe that such and such judgment is true or false. Descartes describes this sort of evidence as “taught by nature”; in other words, experience combined with our natural faculties produces certain beliefs in us about the world. Descartes distinguishes sort of natural belief from what is shown to us by the “light of nature.” Judgments based on the former, like the apparent size of the sun, are susceptible to error; judgments based on the latter, like the “I am” which issues from the “I doubt”, can not be doubted. The certainty of the latter judgment is based on the absolute proximity of the “I” to itself, compared to the uncertain distance dividing the self and what exists outside it.
Fifth, Descartes introduces the possibility of gradations of objective reality among the ideas, and names the idea of the infinite, eternal, supreme deity as the idea with the most objective reality. Descartes is skeptical that judgments “taught by nature” can lead to justified true beliefs. However, he suggests that certain ideas “contain within themselves more objective reality than those which represent only modes or accidents.” He could be referring here to something like Aristotle’s primary substance, the idea of which forms the basis for many more ideas as derivations. The idea of God would be primary in this way, except to the absolute extent that it is the only idea whose thinking guarantees its ideatum. Whereas Descartes separates lesser ideas, as in-themselves incapable of truth or falsity, from judgments, with the idea of God, its objective reality impresses him to the point that the idea itself functions as a judgment. Namely, that God exists. Descartes insists this is proved in the strong form of proof: judgment as shown by the “light of nature” (not “taught by nature”) and therefore impervious to doubt.
Sixth, Descartes expands on the proof. The cause must possess at least as much objective reality as the effect. Arguing from effect to cause, Descartes insists that the objective reality of idea of God guarantees the existence of God, irrespective of all the doubt we entertain of lesser things outside us. Something (the idea of God) can not come from nothing; what is more perfect cannot come from what is less perfect; and the ideal history of a thing’s origin cannot regress ad infinitum. Therefore, from the objective reality of the idea of God, it follows that there is a God, He is as perfect as the idea or more so, and He is the absolute genesis of the idea of God in containing formally all the reality that is in the idea objectively.
Seventh, Descartes elaborates on the objective reality of the idea of God. We cannot be the origin of any thought that thinks an objective reality beyond that which we may claim for ourselves. The idea of God thinks the infinite; but we are finite; therefore we can not be the origin of the idea. The objective reality of the idea of God consists in its resistance to our powers of thought, and independence from the powers of our thought. It is a thought that we would not think unless it were given to us from the outside because there is no resource within the internal subjective workings of our thought that could produce it. Descartes describes the resistant and independent character of the idea of God as its “greatness.”
Eighth, Descartes distinguishes the clear and distinct idea of God from the clear and distinct idea of corporeal things. What is clear and distinct in the ideas of corporeal things could have been borrowed from the finite self: substance, duration, number, and so forth, can be derived from powers of finite thought. The idea of infinity and all the divine attributes cannot.
Ninth, Descartes prioritizes the perception of the infinite over the perception of the finite. The idea of the infinite is not simply the negation of finitude – the idea of the infinite is a positive thought. As positive and more perfect, it is prior to any idea or perception of the finite.
Tenth, Descartes continues to elaborate on the objective reality of the idea of the infinite. Our lack of comprehension of the infinite is not an argument against its objective reality. Even if we lack comprehension of God, we are able to comprehend the perfection of lesser things, the gradation of perfection, and that a greater perfection cannot come from a lesser perfection: taken together, we can trace the genesis of objective reality along a line which reaches beyond the threshold of the level of our own perfection, to a beyond of necessity that we fail to understand of necessity.
Finally, Descartes concludes that a finite being could not exist without a being of greater perfection from which the finite being originated. A finite being evidently exists, therefore an infinite being exists, as a creator, and the innate idea of the infinite is the mark of that infinite being. Descartes runs through assorted arguments which, in various ways, insist that the objective reality of the idea of the infinite could never have originated from me as a finite being. The crux of the argument rests on whether or not the reader is impressed by the notion of an idea that self-determines its own reality beyond the reality of all other ideas. It’s not enough for Descartes’ argument for the reader to simply have faith in God; he also wants us to see, to assent to, the innate quality of the idea of the infinite in us. It is there within us to be discovered in our own experience, even though the objective reality of the idea of God will lead us to a certainty (certainty of His existence) beyond the normal certainty of judgments drawn from average experience. Descartes want us to see that the self-determining reality of the idea of the infinite is a move justified by the “light of nature,” not by what experiences provides us as “taught by nature.”
Having shown by the “light of nature” that God exists, Descartes next step in the fourth meditation will be to show that God is not a deceiver.