Kant starts the Groundwork of the Metaphysics of Morals by emphasizing the importance of separating the a priori or rational part of moral philosophy from the a posteriori or empirical aspects. Indeed, he reserves the term moral philosophy for the rational part. He writes ‘ethics … the empirical part might be given the special title practical anthropology, the term moral philosophy being properly used to refer just to the rational part’. Throughout his writings in both theoretical and practical philosophy the distinction between what is a priori and what is a posteriori is given paramount importance. We need to separate that which has its source a priori from its application to, for example human beings.