According to Anselm, God, as the greatest thing that can be thought, is accordingly the most perfect thing as well. Pure perfection, as a philosophical concept, devolves on the possessor all positive properties. The nature of the property of pure perfection determines whether this or that attribute, such as goodness or badness, is included or not. For Anselm, “existence” is the property par excellence, because it is the greatest perfection. God, who is perfect and lacks no positive properties, is the only conceivable idea such that the idea corresponds to the existence of God— as in the case that the idea of God exists; hence, God exists.
Anselm’s proof for the existence of God marks out two possible types of existence: first, existence in the understanding, and second, existence in reality. If an object has existence in the understanding, then it exists as an idea, but if an object exists in reality, then the idea of that object refers to an object qua object. A desk, for example, exists in both the understanding and in reality, whereas mythological beasts exist only in the understanding. According to Anselm’s argument, it is more perfect to exist in both reality and in the understanding, rather than in only one. Existence in both reality and the understanding does not imply perfection qua perfection, or even actual existence. In the case of God, either God exists in the understanding but not in reality, or God exists in both reality and the understanding. In the former instance, God is a perfect thing that lacks something, namely existence; hence, God is an imperfect perfect thing, which is contradictory. God, however, is necessarily perfect, and thus the idea of God in the understanding as “that which nothing greater can be thought” must refer to an object that has actual existence if it is to also be the possessor of all possible perfections.
In his Critique of Pure Reason, Kant addresses the claims of Anselm’s “ontological” proof for the existence of God. According to Kant, Anselm’s argument fails to gain its objective as an analytic or a priori proof for God’s existence because “being,” or existence qua existence, is not a real predicate in the statement, “God exists.” Possessing an external correlate (being) is not really a property since sentences can be constructed where being appears to be a real predicate, but is not, such as in the case of “God exists.” Kant states that not all a priori knowledge is vacuous, and this leads him to distinguish between two types of judgments, or propositions: analytic and synthetic. Analytic judgment/propositions are those in which the predicate in a proposition is contained in the subject (red is red, or all bodies are extended), and are merely explicative. The converse of an analytic proposition is a self-contradiction, and thus all analytic judgments are known a priori. Analytic proposition/judgments refer to those characteristics that do not extend or supplement knowledge about the world, since knowledge independent of experience cannot tell you anything about experience; thus, the concepts contained in analytic judgments refer to the logical structure of the mind alone, not any rationality underlying the contingencies of perception). Synthetic judgment propositions are those in which the predicate of a proposition is not contained in the subject (triangles are blue, or all bodies have weight), thus synthetic judgment/propositions are ampliative.
In Anselm’s proof for the existence of God, what is being predicated of God is neither analytic nor synthetic. Existence cannot be a property of anything because it adds nothing to the subject— if the predicate “existence” is added to the subject “Pegasus,” existence as a predicate is only added to the subject as an idea, and cannot bestow real existence on the subject if it doesn’t already exist. On the other hand, synthetic predicates add something specific to the object it’s predicative of; or in other words, it adds content to the subject. For example, the statement, “the triangle is red” predicates redness of the subject triangle. If existence were a predicate, then it would be adding content to what is predicated of it, but it does not. In the case of Anselm’s proof, the claim that God has existence as a correlate is an analytic judgment. As an analytic judgment, the proposition, “God exists” is the sole example of an analytic judgment that establishes the existence of something, that it is true by definition, and that what is predicated of the subject is true. Contrary to this claim, Kant claims that the predicate of existence only appears to be predicating something of the subject, when in fact it is not. Because the predicate “existence” is put into the form of a sentence, it only appears to prove what it claims, but in fact assumes what it sets out to prove, rather than proving it.