Reason.

J.I. Packer has written:

“‘Reason’ means reasoning, as ‘faith’ means believing and trusting. The first is the mark of men, as distinct from beasts; the second, of Christians, as distinct from unbelievers. As there can be no faith without thinking (for truth must be known before there can be trust), so for the Christian there should be no thinking without faith (for thoughts that do not express faith are sin¹). The Christian’s intellectual vocation is to think about all things in such a way that his life of thought is part of his life of faith and homage to God. Whereas the non-Christian is led by faithless reason, the Christian should be guided by reasoning faith.”

(Packer, J.I. “Fundamentalism” and the Word of God, Eerdmans, 1972, p. 128.)
¹ Cf. Rom. xiv. 22, 23.