Kant's Refutations of the Proofs of the Existence of God
There are three types of proof for the existance of God:
The Ontological Proof:
God is the most perfect conceivable being. Existence is more perfect than non-existence. God by definition exists.
The Cosmological Proof:
Everything contingent must have a cause. If this cause is also contingent, then it too must also have a cause. This chain of causes and effects must have a beginning - a necessary cause. This necessary cause must be God.
The Physico-Theological Proof:
Observations about the particular constitution of the sensible world provide proof of the existence of God.
As we saw earlier in the CPR, there are two types of judgment: analytical and synthetic. The example that Kant used for an analytical judgment was "a triangle has three angles" (p.564). This is obviously true, because by definition a triangle must have three sides - all that one would need is the knowledge of the definition of triangle in order to see that the predicate (three angles) is contained in the subject (triangle). The ontological argument claims to be analytical in that it proves the existence of God because existence is contained in the definition of the word God. An analytical judgment, like the one given above, does not imply necessity - the words "if" and "then" are implied. The statement could also be read: "If there are triangles, then they have three angles" and could be negated without contradiction "If there are no triangles, then they do not have three angles". However, it is claimed that the ontological argument is the single exception to this rule. If existence is in the definition of the word "God" then: "If there is no God, then no God exists" is a contradiction and would be like saying: "If there are no triangles, then the concept of a triangle does not contain three angles". As soon as a person has admitted that he/she has a concept of "God", by this argument, then that person has also admitted God's existence (in the same way that admitting you have the concept of a triangle is, is to admit that a triangle has three angles).
Like most of Kant's topics of discussion, it is difficult to fully grasp the entirety of what is being said. However, unlike most other cases, this time I don't think it's entirely his fault. In fact, his refutation to this deceivingly simple argument is (perhaps deceivingly) actually quite simple. Existence cannot be a predicate. In the sentence "Thomas is the teacher", the word "is" is not the predicate, but it is the word that implies existence. Therefore, the sentence "God is" does not contain a predicate. A predicate is a word that ads a quality to the concept of the thing it is applied to -- existence is not a quality.
The cosmological argument, according to Kant, "is too well known for it to be necessary to expound it in detail here" (P570), so...