Neither shall I waste time at this stage by discussing in what sense it is permissible to speak of “proof” of so transcendent a reality as the Divine existence. We remember here the saying of Jacobi, that a God capable of proof would be no God at all; since this would mean that there is something higher than God from which His existence can be deduced. But this applies only to the ordinary reasoning of the deductive logic. It does not apply to that higher kind of proof which may be said to consist in the mind being guided back to the clear recognition of its own ultimate pre-suppositions. Proof in Theism certainly does not consist in deducing God’s existence as a lower from a higher; but rather in showing that God’s existence is itself the last postulate of reason—the ultimate basis on which all other knowledge, all other belief rests.
James Orr The Christian View of God and the World. 9th edn 1908. Lecture III p 94-5.