Kant's Critique of Pure Reason
TRANSCENDENTAL LOGIC, SECOND DIVISION: TRANSCENDENTAL
DIALECTIC
BOOK II: THE DIALECTICAL INFERENCES OF PURE REASON
Chapter III: THE IDEAL OF PURE REASON
Section 3: THE ARGUMENTS OF SPECULATIVE REASON IN PROOF OF THE EXISTENCE OF A SUPREME BEING (p. 495)
We would not be persuaded that the primordial being is real (God) if we were not pushed to find the unconditioned.
If we admit something exists, we must also admit there is something which exists necessarily.
Reason looks around for a concept that is compatible with absolute necessity.
It finds the primordial being which is the condition of all possibility. Where can we more suitably locate the ultimate causality than where there also exists the highest causality?
But, it does not follow that a limited being is incompatible with absolute necessity, although we cannot infer its necessity from the universal concepts we have of it.
Just as in the hypothetical syllogism we cannot say that where a certain condition does not hold, the conditioned also does not hold.
Thus the argument fails.
There are only three possible ways of proving the existence of God by means of speculative reason:
Physico-theological: begins with determinate experience and ascends from it in accordance with the laws of causality to the supreme cause outside the world.
Cosmological: begins from experience of existence in general.
Ontological: abstracts from all experience, and argues completely a priori from mere concepts to the existence of a supreme cause.