In the Wolffian philosophy, God is the most perfect being. The most perfect being combines all the realities that can be combined in one thing, and the realities combined are the greatest realities that can be in a thing. A most perfect being is possible, because there are only realities and no negations in it. Since actuality is a reality, God is actual. Thus the notion of God is that of the ens realissimum, the most real being, as described by Kant as the ideal of pure reason.
An ideal is an individual thing determined or determinable by an idea of reason. Kant held that Plato's forms are ideals, i.e., ideas in God's understanding which are intuited by God, and which are the most perfect of a given kind. It is the archetype of which all things of that kind are copies. On a more mundane level, the Stoic's Wise Man is an ideal of humanity. This ideal can have some moral force, though it is not a real object. Indeed, Kant claimed that attempts to describe an ideal in works of fiction lead to absurdity. Reason has its ideal, which is transcendent, in that the object cannot be found in experience.
An individual is thought through predicates. A concrete individual is determined with respect to every pair of possible opposing predicates. That is, for every predicate F, an individual is either F or non-F. (This is called the "principle of complete determination.") We can think of the quantifier over predicates as having as its domain the "sum-total of all predicates of things." A thing is only possible insofar as it draws its predictes from this domain. The "material for all possibility" is hence a "transcendental presupposition" of the principle of complete determination. The actual determining of a thing in cognition, if it were complete, would require going through all possible predicates: something we humans cannot do. We do not know exactly what all the possible predicates are. But when we form an idea of the sum of all these predicates, we represent an individual, though only an ideal. Since negations are only derivative from their positive counterparts, the idea is that of an "All" of being, a being which contains every positive predicates.
Of course, such an ideal could only be a thing in itself. Reason represents it only as a ground of the properties in limited beings. "The ideal is, therefore, the archetype (prototypon) of all things, which one and all, as imperfect copies (ectypa), derive from it the material of their possibility, and while approximating to it in varying degrees, yet always fall very far short of actually attaining it" (A578/B606). We have no knowledge of the existence of such a pre-eminent being. This being is not composed of limited being, but is their ground. We can find all the traditional properties of God in the ideal of pure reason, so it is the object of transcendental theology.
Transcendental theology is not a legitimate science, as it goes much further than the idea allows. We cannot say that it determines an object, but only that it can be of use in detemining given objects. To do so is an illusion, which Kant traces to a fact about objects of experience, namely, that they presuppose the sum total of experience, of which they are limitations. If we omit the qualification that these objects are appearances, "we treat the empirical principle of our concepts of the possibility of things, viewed as appearances, as being a transcendental principle of the possibility of things in general" (A582/B610).
Reason seeks the primordial being to find satisfaction in the unconditioned. In experience we find only contingent beings, and reason will not rest until it finds a being which is unconditionally necessary as its support. From the requirement that it find a necessary being, it seeks for something that could be thought as necessary, "whether or not its necessity be comprehensible, that is to say, deducible from its concept alone" (A585/B613). The ens realissimum fits the bill, since it depends on nothing else, but is the source of all possibility. No other concept measures up to it in this respect, so we cannot know whether its object is unconditioned. "The concept of an ens realissimum is therefore, of all concepts of possible things, that which best squares with the concept of an unconditionally necessary being; and though it may not be completely adequate to it, we have no choice in the matter, but find ourselves constrained to hold to it" (A 586/B614). Kant held that people in all ages have had a glimmer of monotheism, being persuaded that there is a supreme cause of things which is absolutely necessary.
Threre are three ways possible of proving God's existence: from the specific constitution of the world (design, or physico-theological, argument), from the fact of existence in general (cosmological argument) and from mere concepts alone (ontological argument). Kant begins his criticism of these proofs with the ontological argument, on the grounds that this best represents the true aim of reason. The other arguments depend on experience, and hence could only serve a supporting role.
It might be held that the ens realissimum is a concept which contains existence in it. The concept is that of something possessing all reality, and 'all reality' includes existence. To reject existence is to reject the possibility of the thing.
Kant's response is that it is illegitmate to smuggle existence into the concept of a thing which is being considered solely with respect to its possibility. He posed a dilemma: the judgment that this thing exists is either analytic or synthetic. If analytic, it is true because the concept of existence is already found in that of reality, for the concept in question is that of the sum of all reality. But this only means that the word 'reality' has been used to mean something more than 'possible predicate': it means existence, and the question has been begged. If synthetic, the judgment can only be true on the basis of experience, and hence that this thing exists can be known only a posteriori.
Here Kant made the famous claim that 'being' is not a real predicate. There is a confusion between logical predicates and real predicates. A logical predicate is any predicate one pleases, while a real predicate is one which enlarges the concept to which it is attached. Being is only a logical predicate. When we use 'is' as the copula, it indicates that the subject and predicate belong together. Taking the subject alone, and saying that it is, one has not enlarged the concept of the subject at all, but only "posited" it as object. In a later passage, Kant put it this way: "the knowledge of the existence of an object consists precisely in the fact that the object is posited in itself, beyond the thought of it" (A639/B667). One thinks in the subject no more than what one thought in it when conceiving it merely as a possible object. "A hundred real thalers do not contain the least coin more than a hundred possible thalers." If it did, then the original concept was inadequate. "If we think in a thing every feature of reality except one, the missing reality is not added by my saying that this defective thing exists. On the contrary, it exists with the same defect with which I have thought it, since otherwise what exists would be something different from what I thought" (A600/B628).
If the thalers do exist, this fact is added synthetically, through experience, to the concept of a hundred possible thalers. This much is obvious for objects of the senses like coins, where we know that they exist only through perception and their connection to it. But we cannot know an object such as God through perception, and we are left without a way of establishing its existence. Indeed, we cannot say that God is a real possibility, that is, something that satisfies the Postulate of possibility, as conforming to the conditions of experience. We can only say that it is logically possible. "Thus the celebrated Leibniz is far from having succeeded in what he plumed himself on achieving -- the comprehension a priori of the possibility of this sublime ideal being" (A602/B630). And Descartes' ontological argument is "so much labor and effort lost," comparable to that of a merchant seeking to improve his financial position by adding zeroes to his bottom line.
This intelligible cause would be "the purely transcendental and to us unknown ground of the possibility of the sensible series in general" (A564/B592). Thus although it is thinkable, such a necessary being is unknown and its existence cannot be proved. This puts the cosmological argument for God's existence in a hopeless position from the start.
One begins the argument with one's self as a contingent being (my own existence is known through experience). My existence, and that of the entire world, is explained through its dependence on a necessary being. So much was argued by Leibniz. But Kant continues the argument by moving from a necessary being to the ens realissimum. The idea of an ens realissimum is brought in by reason only to satisfy its demand for a necessary being, to stop the regress from contigent being to contingent being. The necessary being is not determined by some other being (else it would be contingent) and so is entirely self-determined. Only the ens realissimum is completely self-determined, and so is "the only concept through which a necessary being can be thought" (A605/B633).
Breaking the proof up into two parts, the first experiential and the second a priori, seems to lend it more authority than the ontological argument, for experience provides a "secure foundation" and reason supplies the link to full-fledged being, the ens realissimum. However, the appeal to experience is superfluous, since the a priori argument states that a most real being is a necessary being. But this (if sound) would secure the necessary existence of the most real being, which is just what the ontological argument attempts to prove. "If I say, the concept of the ens realissimum is a concept, and indeed the only concept, which is appropriate and adequate to necessary existence, I must also admit that necessary existence can be inferred from this concept" (A607/B635). Thus the cosmological argument is deceptive in its appeal to experience.
There are other problems with the argument: (1) Inferring a cause for contingent beings is legitimate only if the cause is confined to the world of experience. "The principle of causality has no meaning and no criterion for its application save only in the sensible world" (A609/B637). (2) The inference to a first cause is not even legitimate witin the world of experience. (3) To go beyond the world of experience gives only a false impression of completeness. The appeal is to something quite empty. (4) The ens realissimum is not even a real possibility, as was discussed above.
The underlying problem is that we cannot find any object whose existence is necessary, yet reason is compelled to claim that there is a necessary being as a condition for what exists. "I can never complete the regress to the conditions of existence save by assuming a necessary being, and yet I am never in a position to begin with such a being" (A616/B644). This conflict can only be resolved if the principles (think something as necessary, think no particular thing as necessary) can only be viewed as heuristic and regulative, not objective. The thought of a necessary first ground brings unity to experience, the other offers a warning not to close off the series of causes. This requires that we can only think a necessary being outside the world, as if it were the cause of the world. Transcendental illusion results from regarding the regulative principle as constitutive.