Here then we see philosophy brought to a critical position, since it has to be firmly fixed, notwithstanding that it has nothing to support it in heaven or earth. Here it must show its purity as absolute director of its own laws, not the herald of those which are whispered to it by an implanted sense or who knows what tutelary nature. Although these may be better than nothing, yet they can never afford principles dictated by reason, which must have their source wholly a priori and thence their commanding authority, expecting everything from the supremacy of the law and due respect for it, nothing from inclination, or else condemning the man to self-contempt and inward abhorrence.

Fundamental Principles of the Metaphysic of Morals