The question ‘Why ought we to do what we think we ought to do?’ was said by H.A. Prichard to be an improper question, manifesting erroneous presuppositions about the nature of moral reasoning. Moral rationalists think it a legitimate question, and they maintain that we are rationally justified in acting as we are morally obliged to act. Anti-rationalists deny this claim: specifically, they deny that acting in accord with moral obligation is endorsed by practical rationality. In my dissertation I try to uncover the source of disagreement amongst these three views. I then propose a resolution by appealing to my interpretation of Plato’s version of moral rationalism. I argue that contemporary moral rationalists and antirationalists in fact share the presupposition(s) that Prichard regarded as erroneous and which are not found in Plato. However, Prichard misread Plato as a rationalist of the instrumental persuasion and also offered only the beginnings of a positive view on the nature of practical reason. Therefore he failed to recognize that Plato gives a good positive answer to the original question on which the rationality of morality does not depend upon endorsement from any external source. Even so, Prichard’s limited remarks on the nature of moral deliberation show intimations of a better conception of the nature of moral rationality than is assumed in the contemporary debate between moral rationalists and anti-rationalists. One proponent of a more developed conception of the nature of moral thinking in the spirit of Plato is Iris Murdoch, and from her work I sketch a conception of the relation between thought, words and moral experience that in my view offers a way to understand the rationality of morality that is truer to the phenomena.