The required a priori element in our view of morality

Many philosophers think we can know some moral propositions a priori. Few deny that we can know (4) a priori or the proposition that if it would be wrong not to do something, one ought to do it or other propositions that are obviously true because of definitions. But some philosophers claim we can know more significant moral propositions a priori.

On one familiar view, we can know a priori the fundamental moral principle (or principles), e.g., the principle that one ought to perform the action that has the overall best consequences, or the principle that one ought to act in accordance with virtue, or whatever the basic moral principle really is. Some hold we can know the principle via analysis or because it defines the moral term it is about. We can then know every day moral propositions about particular actions or types of actions by inferring them from the fundamental principle in conjunction with empirical facts.

However, some theorists claim we can know many more moral propositions a priori, in particular, propositions that are not so closely tied to the meaning or reference of moral terms. Others go farther, claiming that most or all moral claims can be known a priori, or even that moral claims can be known only a priori.