In the second part of the “Transcendental Logic” (“Transcendental Dialectic,” chap. 3, “The Ideal of Reason”), Kant presented a systematic critique of metaphysical proofs of the existence of God. Each element of this critique was inspired by an idea central to Kantianism. For Kant, the only role of the principles of reason is to regulate experience. So it's a fallacy to rely on them to demonstrate the existence of a being who, by definition, is outside of experience. Reason ties together and organizes the phenomena we perceive; but God is an absolute and doesn't exist within the phenomenal world.

Let's back up and examine the proofs we've just discussed. The first assumes that within us there exists an innate idea of perfection. We've already refused to accept several of the proposed principles of reason, on the grounds that they have no foundation in experience. Yet this proof rests on one of these contestable principles, that the cause has as much reality as the effect. This principle assumes that the effect derives from and is adequate to the cause. But this is a mathematical conception of things that's out of touch with reality. Cause and effect needn't be of the same type – the effect might have quite novel qualities and characteristics. For example, the reality of water is entirely adequate to its cause – oxygen and hydrogen combined by the influence of electricity – but the effect and cause are heterogeneous.