moral rationalism the view that the substance of morality, usually in the form of general moral principles, can be known a priori. The view is defended by Kant in Groundwork of the Metaphysic of Morals, but it goes back at least to Plato. Both Plato and Kant thought that a priori moral knowledge could have an impact on what we do quite independently of any desire that we happen to have. This motivational view is also ordinarily associated with moral rationalism. It comes in two quite different forms. The first is that a priori moral knowledge consists in a sui generis mental state that is both belief-like and desire-like. This seems to have been Plato’s view, for he held that the belief that something is good is itself a disposition to promote that thing. The second is that a priori moral knowledge consists in a belief that is capable of rationally producing a distinct desire.
Rationalists who make the first claim have had trouble accommodating the possibility of someone’s believing that something is good but, through weakness of will, not mustering the desire to do it. Accordingly, they have been forced to assimilate weakness of will to ignorance of the good. Rationalists who make the second claim about reason’s action-producing capacity face no such problem. For this reason, their view is often preferred. The best-known anti-rationalist about morality is Hume. His Treatise of Human Nature denies both that morality’s substance can be known by reason alone and that reason alone is capable of producing action.
See also AKRASIA, ETHICS, HUME, KANT, MORAL SENSE THEORY, MOTIVATIONAL IN – TERNALISM , RATIONALIS. M.Sm.