The purpose of this paper is to examine Hume's arguments against moral rationalism, which claims that virtue and vice could be ascertained by reason alone and ascribed the major role in moral judgement to the intellect. 'The motivation arguments' and what is called 'the moral discernment arguments' from the sectionsⅡⅲ１& Ⅲⅰ１of Hume's Treatise and his 2nd Enquiry‘s AppendixⅠ are generally regarded as the classical critique of moral rationalism and the statement of an empirical naturalist approach to practical reason. Through them Hume argues that morality cannot be based on reason since reason cannot strictly oppose or endorse any action or motive. Reason's task is the discovery of truth and falsehood. Reason has no power to motivate the will by itself. Sentiments or passions correspond to judgements of virtue and vice respectively. All exciting reasons or motives for action, are seen as grounded in passions, which supply the ultimate end of action. Reason's function is restricted to providing information on the means to satisfy these ends. Reason can only justify an act in the sense of recognizing it as being a viable means to an end which is determined by the passions. Hume's conclusion from his arguments against moral rationalism is that reason has no power to motivate the will by itself and moral distinctions are not based on the exercise of reason but rather are the result of our inner sentiments. Hume, therefore, claims that moral distinctions cannot be derived from reason and morality is more properly felt than judged of and also reason is the slave of the passions.