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.
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. 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. 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. Kant begins his criticism of these proofs with the ontological argument, on the grounds that this best represents the true aim of reason.