William Wainright on Kant and Transcendent Morality"Kant, God, and Immortality" in Religion and Morality (Ashgate Publishing: 2005), p. 7.
The trouble with acting solely on the basis of natural incentives like sympathy is therefore this. The maxims which are guiding our actions are derived from desires which aren’t shared by all (possible) rational beings, and thus can’t be regarded as expressions of pure moral reason. ¶ Why does Kant adopt this position? A person’s emotions, feelings, and inclinations are part of his or her biological inheritance. However admirable they may be, acts that are only expressions of feeling and inclination are acts of human animals, of beings caught up in the web of nature, locked into the system of natural causes and effects. When we act because we see that something is right, however, our behavior is an expression of our reason and will, of those aspects of ourselves which transcend nature. ¶ Two “worlds” or realities must be distinguished. The phenomenal world or world of appearances discloses itself in sense perception and is investigated by science. It includes observable substances, qualities, and events, and theoretical entities like subatomic particles which science postulates to explain them. “Behind” the world of appearances lies the noumenal world — reality as it is in itself, and not as it manifests itself to us. This world is inaccessible to theoretical reason and is therefore, in the strict sense, unknowable. But human beings belong to both worlds. As parts of nature, we are members of the phenomenal world, and our behavior can be explained in terms of natural causality. As free and rational beings, we are members of the noumenal world, and our actions are self-determined.