Descartes's fifth Meditation argument for God's existence relies on an untenable notion that existence is a perfection and that it can be predicated of God. I shall first explain what Descartes's argument for God's existence is, and then present his argument in propositional form. I will then attempt to support the argument that existence is neither a perfection nor a predicate of God.
In our thoughts we apprehend ideas of things. These ideas may reside entirely within our thoughts or they may exist independent of our considerations of them (Descartes 143). Descartes argues that the idea of God is that He is infinite substance "[eternal, immutable], independent, all-knowing, all-powerful" to which nothing more perfect can be imagined (Descartes 149; 151). Descartes defines the more perfect as "that which contains in itself more reality" (Descartes 146), so that there are gradations of perfection beginning with the subjective phantasms, such as a chimera, and culminating with the most perfect being in God Himself. Thus, because our idea of God is one of absolute perfection, and existence contains more reality than nonexistent thoughts alone, God exists. Descartes's argument can be represented logically as:
(1) In our thoughts we experience an idea of the most perfect being.
(2) Existence in reality is more perfect than existence in our thoughts alone.
Therefore, (3) the most perfect being exists in reality.
Descartes wishes to argue that existence is a perfection and hence it belongs to those characteristics of the divine nature. However, a thing cannot possess a characteristic unless it first exists. In his reply to Descartes's argument, Gassendi complains that "that which does not exist has neither perfection nor imperfection, and that which exists has various perfections" (Plantinga 46). Existence, therefore, should more properly be thought of as a prerequisite for perfection and not a perfection in and of itself. Descartes disagrees however, and in his reply to Gassendi he argues that existence is necessarily predicated of God because existence is a part of the true essence of any perfect being (Plantinga 49). For Descartes, it is not possible for us to possess the idea of a most perfect being if this being lacks the most important characteristic of existence. If God did not exist then He would not be the most perfect being, but we clearly have the idea of the most perfect being so therefore He must exist. The problem with this notion, however, is that Descartes begs the question by building into premise (2) the concept of a perfect being which has yet to be demonstrated. In order to demonstrate God's existence, Descartes should not assume, or presuppose, that which he is attempting to conclude. But by predicating the existence of God in (2) he has already concluded that which is later restated in the conclusion. In effect, the follower of Descartes's argument is tricked, for if he or she agrees with the foundational premises for the sake of the argument (that existence is predicated of a most perfect being), then there is no choice left but to conclude that God exists.
Further, predicating the existence of a most-perfect being based upon the attributes that this being is believed to possess fails to provide existence to that being. We could add many more characteristics to Descartes's list of divine attributes (Good or Just for instance), but these attributes do not predicate actual existence, rather, they describe what such a being should be like. Kant argues that existence cannot be predicated of a thing at all, and that no matter how many "predicates we may think a thing [has]. we do not make the least addition to the thing when we further declare that this thing is" (Kant 504-5). Descartes's suggestion that "God is x" relies on the grammatical construction of the copula is while remaining unclear about the logical use of the is of existence. It is true that the object of the is of predication must necessarily exist in order to form a coherent sentence. Sentences such as "God is eternal" predicate eternity of God; grammatically the subject of the verb must necessarily exist. But it is not at all clear that the is of existence can be understood the same way. When we say "God is," logically we are really saying "for all x, if x is a God, then x exists" (Barnes 51). In this understanding of exists, the divine attributes that are predicated of God (including the attribute of existence that Descartes wishes to predicate of God) can only describe what God should be like provided He actually exists. Therefore, when Descartes predicates existence of God he is uttering a grammatically coherent sentence, but a very confusing logical proposition. To say "God exists" is to say "for all x, if x is a God, then x is existent" which is another way of saying "if God exists then He exists." Rather than predicate actual existence to God, this logical proposition merely restates the problem.
I have argued that existence should not properly be seen as a perfection, rather, existence is a prerequisite of any being who is then capable of being either perfect or imperfect. Further, Descartes's argument builds into his premises the conclusion that he is trying to demonstrate, namely that a most-perfect being must exist in order to be a most-perfect being. Last, predicating the existence of God as a divine attribute seems to be unhelpful in addressing His actual existence. Descartes needs to arrive at God's existence through empirical means that do not rely on a restatement of the problem in the form "if x then x" as a solution to God's actual existence.