The Role of Reason
Kant’s system of ethics attempts to derive the moral law from reason. Immorality, according to Kant, involves inconsistency, and is therefore irrational.
This feature of Kantian ethics has two important implications.
The first implication of Kant’s use of reason to ground morality is that it provides a response to the egoist. Egoism holds that we ought only to act in our own self-interest. Most philosophers reject egoism, but it is notoriously difficult to give an adequate justification for doing so. Kant’s theory provides such a justification: egoism is irrational, and so can be criticised on that ground.
The second implication of Kant’s use of reason to ground morality is that it explains the scope of morality. Rationality, for Kant, is definitive of human nature; it is universal among human beings. All human beings, then, because that have the capacity to be rational, ought to be moral. Other animals, lacking this rational capacity, are not subject to the moral law, and therefore cannot be judged by it.