In his 3rd Meditation, Descartes argues for the existence of God and tries to escape metaphysical solipsism.  Ultimately, Descartes thinks he succeeds in his endeavors through an argument he constructs from what he learns in an examination of the necessary conditions for his ideas.  Critics of Descartes argue that he does not succeed in proving the existence of God and escaping metaphysical solipsism since Descartes should not be able to trust his reasoning, and one can only have a negated idea of the finite and not a positive idea of the infinite.  Although Descartes’ argument fails within the context of his thought experiment’s extreme skepticism, it succeeds outside of this context since we can and must have a positive idea of the infinite.  We must have a positive understanding of both the negation of the finite and the infinite to distinguish between the two.  Furthermore, it seems more logical to argue that the finite is a negation of the infinite, not vice versa, indicating we have prior knowledge of the infinite to identify the finite.

            After re-affirming what he already knows with certainty due to his previous meditations, i.e. that he is a thinking thing whose ideas (mental representations) do exist in his mind, Descartes tries to argue for the existence of God and escape metaphysical solipsism by examining the origin of his ideas and the necessary conditions for his ideas (Descartes 47.2-48.1 & slides).  His first method fails, but he believes his second method succeeds.  Descartes begins his second method by stating his metaphysical conservation principle: “Now it is indeed evident by the light of nature that there must be at least as much [reality] in the efficient and total cause as there is in the effect of that same cause” (Descartes 49.2).  Stated simply, the reality of a cause must be equal to or greater than the reality of its effect.  Furthermore, Descartes makes an important distinction between the natures of causes and effects: “For just as the objective mode of being belongs to ideas by their very nature, so the formal mode of being belongs to the causes of ideas, at least to the first and preeminent ones, by their very nature” (Descartes 50.1).  Thus, according to Descartes, his objectively real ideas must be caused by beings with formal existence. 

            Having established his metaphysical conservation principle and clarified the natures of causes and effects, Descartes begins assessing his ideas and realizes that all of his ideas can be formally traced back to himself with the exception of his idea of God since this idea pertains to an infinite substance while he is finite (Descartes 50.2-51.2).  He then concludes that his objectively real idea of God must be formally caused by God which means God must exist (Descartes 51.2).

            A classic criticism of Descartes’ argument is that it fails in the context of his extreme skepticism since he would not be capable of trusting his reasoning (slides).  In his 1st Meditation, Descartes tears down all of his sources for obtaining knowledge which could possibly mislead him in order to establish an indubitable foundation for knowledge.  These sources include his sense organs because of their potential for misleading him with regards to small and distant objects and the possibility that he could be dreaming, making even his most immediate sense perceptions potentially false (Descartes 41.1-42.1).  Finally, Descartes even realizes that he cannot trust his understanding of the abstract sciences, such as geometry and arithmetic, since an evil demon could have given him a nature that could be deceived in these matters: “To be sure, I did decide later on that I must doubt these things [abstract sciences], but that was only because it occurred to me that some God could perhaps have given me a nature such that I might be deceived even about matters that seemed most evident” (Descartes 48.1).  As a result of this extreme skepticism in pursuit of what can be indubitably known, Descartes cannot even trust his own reasoning.  By the time he arrives at his 3rd Meditation’s proof of the existence of God, Descartes’ critics point out that he is still potentially at the mercy of a deceptive evil demon, leaving his reasoning still untrustworthy.  Therefore, Descartes’ critics argue that his argument for the existence of God in his 3rd Meditation fails due to being the product of a dubitable source of knowledge.

            Another criticism of Descartes’ argument is that we cannot have a positive idea of the infinite (God) like his argument requires, but only a negated idea of the finite (slides).  In fact, to Descartes credit, he acknowledges this criticism in his Meditation: “Nor should I think that I do not perceive the infinite by means of a true idea, but only through a negation of the finite” (Descartes 51.2).  Descartes himself realizes that in order for his proof to succeed, one must have a positive idea of the infinite and not just an idea that involves the combination of the concepts of “not” and “finite” (slides).  Those who find this criticism compelling rightly point out that we have many positive ideas of finite things, such as people, colors, and other entities in the finite world in which we live.  However, they deny, with a formidable amount of reason, that we have anything with which to grasp a positive idea of the infinite, making any claim that one has a positive idea of the infinite an illusion and really just the negated idea of the finite.

            Within the context of his extreme skepticism, Descartes’ argument does fail.  Descartes clearly uses his reasoning skills to formulate his proof for God’s existence.  However, he should not be able to trust his reasoning at this point due to still being potentially under the deceptive power of an evil demon.  Therefore, Descartes’ critics succeed in showing that, in the framework of his thought experiment, his argument for the existence of God in his 3rd Meditation fails.  But outside of the context of his extreme skepticism, Descartes does successfully demonstrate the existence of God since we can and must have a positive idea of the infinite.

            Although the second criticism of Descartes’ argument, that we can only have a negated idea of the finite, holds much merit, we can and must have a positive idea of the infinite because (1) we must have a positive idea of the infinite to distinguish between the positive idea of the infinite and the negated idea of the finite, and (2) it seems more logical for it to be argued that the finite is a negation of the infinite rather than vice versa.  First, in order to distinguish between two things, we must have a positive idea of both of those things.  For example, suppose there is a colorblind person who can only see black and white, and can never open their eyes due to a birth defect.  This person would be able to identify blackness when in places with no light, whiteness when in places with light, and thus distinguish between the two colors because he or she would have a positive idea of each of them.  In contrast, suppose there is a completely blind person whose mental image has only ever been and always will be black.  This person would only have a positive idea of black.  As a result, if this person were asked to distinguish between black and white, what could he or she say?  How could he or she say that white is not black without knowing what white is in the first place?  Therefore, those who criticize Descartes by arguing that we can only have a negated idea of finite and not a positive idea of the infinite actually prove that we do have a positive of idea of the infinite by distinguishing between the two ideas.

            Secondly, it seems more logical to argue that the finite is a negation of the infinite as opposed to arguing that the infinite, or our understanding of the infinite, is a negation of the finite.  Indeed, it seems that our understanding of the finite is dependent on our prior understanding of the concept of the infinite.  Descartes makes this point in response to the criticism that we can only have a negation of the idea of the finite: “On the contrary, I clearly understand there is more reality in an infinite substance than there is in a finite one.  Thus the perception of the infinite is somehow prior in me to the perception of the finite” (Descartes 51.1).  To recognize that something is limited and thus finite, it seems someone must first understand the infinite.  Otherwise, they would not have anything to compare a finite thing to in order to recognize its finite nature.

            Descartes presents a unique argument for the existence of God in his 3rd Meditation which brilliantly utilizes the necessary conditions for ideas.  Unfortunately for Descartes, his critics rightly point out that his argument fails within the context of his thought experiment due to his reasoning being subject to the deception of an evil demon.  However, independent of his thought experiment, his argument for the existence of God does succeed since, contrary those who argue that we can only have the negated idea of the finite, we can and must have a positive idea of the infinite.  In order to distinguish between the negated idea of the finite and a positive idea of the infinite, we must have a positive understanding of both ideas.  Furthermore, it seems more logical to argue that the finite is a negation of the infinite rather than the other way around.  As Descartes himself argues, it seems that we first need to understand the concept of the infinite to recognize something as not infinite or finite.