Descartes’ Ontological Proof for God’s Existence

As Thomas Aquinas enters into his arguments for the existence of God in his Summa Theologica he begins by critiquing one of the most famous of all arguments: Anselm’s ontological argument (Q. 2, A. 1). Hundreds of years later, in his Meditations on First Philosophy,Rene Descartes attempts his own version of the ontological argument, whether consciously or unconsciously approaching the proof in a way that seems to address Aquinas’ fundamental critique.

Anselm’s classic argument states that God’s existence is “self-evident” once its definition is known, for God is “that thing than which nothing greater can be conceived”; since it is greater for something to exist actually rather than merely mentally, God must exist actually (Q. 2, A. 1). Anselm’s argument is elegant and rich, for he starts with the mind—what can be conceived—and by this starting point argues that the mind’s ability to conceive has great significance when considering what is “real” and what “actually exists.” For if God is the greatest being in all respects and cannot be conceived as anything other than the greatest, then his existence must be “actual,” for to exist mentally is not greater than to exist actually. While on the surface this may seem paradoxically both persuasive and baffling, Aquinas argues that the proof fails because not everyone conceives of God in this way (Q. 2, A. 1). But even if everyone did, the proof fails because one of its essential premises is assumed but not proven: that “there actually exists something than which nothing greater can be thought” (Q. 2, A. 1). In other words, the gap between “actual existence” and thought is too great. To be able to conceive of something does not necessitate that it actually exists. Thus, in order for the ontological proof to work—according to Aquinas—it must be shown that the very ability to conceive of God necessitates his actual existence, thus bridging the gap between thought and “actual existence.”

This very critique is the gap that Descartes seeks to address in his Meditations. In his “Third Meditation” Descartes concludes that the ability to conceive of God as “infinite, independent, supremely intelligent, supremely powerful,” and the creator of everything necessitates that “God necessarily exists” (31). On the surface, this seems to be almost identical to Anselm’s argument; but the groundwork Descartes lays to lead to this assertion addresses Aquinas’ critique with precision. The argument builds like a mathematical problem, beginning with his emphasis on what can be proven as certainty, which leads to his emphasis on the primacy of the mind.

Descartes begins by separating knowledge into what is “certain” and what is “highly probably opinion” (15), asserting that what can be known or derived through our senses is not on the surface “certain knowledge,” for one may be deceived either by the imperfections of senses, the deceit of a higher power, or by the power of the mind to render dream-like sensations as reality. This doubt concerning the certainty of sensual knowledge runs directly contrary to Aquinas’ proofs for the existence of God: proofs that begin by observation and knowledge of the world outside the mind. For Descartes, what can be known with certainty—at least what kind of certainty has logical priority—is the knowledge of the self, rooted in the knowledge of the mind.

In brief succession, he proves he exists (17), that he is “a thinking thing” (18), and that his knowledge of his own mind is greater than the knowledge derived from his senses, for it is the mental act of “judging” and “the scrutiny of the mind” that is more “clear and distinct” than the external knowledge of objects derived from the senses (21-22). If this be the case, then even if what we experience with our senses is not actually real, our knowledge of how the mind functions and acts toward that stimuli is certain; thus, our knowledge of the mind is more certain than the knowledge of external physical phenomena (24). If this be the case, then, Descartes argues, in order to determine whether anything exists outside of the mind, we must consider the mind itself, specifically how it arrives at certain thoughts and whether any of these thoughts can be said to be produced from something outside the mind.

In “Meditation Three” Descartes brings all these things together to argue for the existence of God. Among the categories of ideas, all ideas are “true” if considered merely as phenomena in the mind, and all the second order considerations are also true as mental activity except judgments, which concern themselves with determining whether “the ideas which are in me resemble, or conform to, things located outside me” (26). Of ideas, there are three kinds: “innate,” “adventitious” (coming outside of oneself), and those invented by the mind (26). From these categories, Descartes proceeds to show that the idea of God cannot be a product of the imagination, but must be innate and can be judged to exist with certainty.

In order to do this, Descartes explains that one cannot assert with certainty that sense perceptions proceed from outside the self, for these could be products of one’s own mind, such as dreams, and even if they did, we cannot be certain that what we sensually perceive accurately corresponds to the external object. Hence, in order to judge with certainty whether a thought corresponds to a reality is not self-produced, one must look inward, to the operations of the mind itself, and judge whether the human mind could be the cause of such a thought. In brief, Descartes asserts that qualities of objects and substances exist “objectively” in the mind, and thus the idea of God as “supreme,” “eternal, infinite, omniscient, omnipotent,” and the creator of everything truly exists “objectively” (although not formally) in the mind (28).[1] Because this idea of God actually exists in the mind, one must be able to account for the cause of such an idea (28). It is here that Descartes brilliantly joins the ontological argument with an argument from causation.

He argues that it is self-evident that “there must be at least as much reality in the efficient and total cause as in the effect of that cause,” such that “something cannot arise from nothing,” and “what is more perfect—that is, contains in itself more reality—cannot arise from what is less perfect” (28). This applies not merely to things that have “formal” reality, but also to ideas, which possess “objective reality” (28). Hence, for the idea of God to objectively exist in the mind, there must be some cause that “contains at least as much formal reality” as contained in the idea of God (28). Even if the objective reality of an idea is imperfect, it cannot arise from nothing, but must be caused from some formal reality (29). It cannot be argued that the objective reality of ideas can come simply from ideas, for “the formal mode of being belongs to the causes of ideas—by their very nature” (29), otherwise it would be an infinite regress: “eventually one must reach a primary idea, the cause of which will be like an archetype which contains formally and in fact all the reality of perfection which is present only objectively or representatively in the idea” (29). Hence, for there to be an objective thought, one must find the formal reality that is equal or greater in perfection: whether that be the mind, the self, or something external to it.

From these premises, Descartes concludes that if “the objective reality turns out to be so great” that the idea cannot reside within the mind formally or “eminently,” then it must follow that it is caused by something outside the self and that thing must also exist (29). While many ideas, such as corporeal things that contain length and breadth and shape, could find its formal source in the self or at least be borrowed from knowledge of the self, this is not so with God. For the idea of God contains objective realities that cannot formally exist in the self nor could they be an amalgamation of what does exist in the self. Since God is “infinite, independent, supremely intelligent, supremely powerful,” and the creator, this objective idea in the mind must be caused by a formal reality that bears those attributes. But the self is none of these things; in fact, the self’s own imperfection speaks to the reality of God’s formal existence as perfect (31).

While some may posit that this idea of God only comes through negation, such as the idea of rest and darkness as the negation of activity and light, Descartes maintains that an infinite substance contains even more “reality” than a finite one, meaning not only that the idea of God cannot be caused by knowledge of the self, but that the idea of God is “prior” to the “perception of the finite, that is myself” (31). Nor can God be a “potential being,” but must be pure actuality (cf. Aristotle), for “the objective being of an idea cannot be produced merely by potential being [. . .] but only by actual or formal being” (32). Thus, because the objective idea of God, who is the Supreme Being, exists in the mind, God must also exist formally and “actually” outside the mind (32).

In this masterful proof, Descartes has addressed Aquinas’ critique of the ontological argument by bridging the gap between ideas and “actual reality.” Whereas Anselm does not explicitly show how the conception of God necessitates his existence, Descartes proves this very thing by showing the priority of the mind as the source of certainty, arguing for the reality of objective ideas, and proving that all objective ideas must have a formal cause, one that is equal or greater to the objective idea it causes. And while all may not be convinced by this, he nonetheless masterfully illustrates the reality of ideas, for all ideas must have a cause.


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