Hume thinks that systems and hypotheses have also perverted our natural understanding of morality. The views of the moral rationalistsSamuel Clarke (1675 1729), Locke and William Wollaston (16601724)are prominent among them. One distinctive, but unhealthy, aspect of modern moral philosophy, Hume believes, is that it allies itself with religion and thus sees itself as serving the interests of popular superstition. Clarkes theory and those of the other rationalists epitomize this tendency.

Clarke, Humes central rationalist opponent, appeals to reason to explain almost every aspect of morality. He believes that there are demonstrable moral relations of fitness and unfitness that we discover a priori by means of reason alone. Gratitude, for example, is a fitting or suitable response to kindness, while ingratitude is an unfitting or unsuitable response. He believes that the rational intuition that an action is fitting has the power both to obligate us and to move us. To act morally is to act rationally. Humes most famous and most important objection to moral rationalism is two-pronged. In Treatise 2.3.3, Of the influencing motives of the will, he rejects the rationalist ideal of the good person as someone whose passions and actions are governed by reason. In T 3.1.1, he uses these arguments to show that moral ideas do not spring from reason alone.

In the first prong of his objection, Hume begins by remarking that nothing is more common than for philosophers, as well as ordinary people, to talk about the combat between reason and passion. They say we ought to be governed by reason rather than passion, and if our passions are not in line with reasons commands, we ought to restrain them or bring them into conformity with reason. Hume counters that reason alone can never be a motive to any action of the will and that by itself it can never oppose a passion in the direction of the will.

His first argument rests on his empiricist conception of reason. As we saw in his account of causation, demonstrative reasoning consists in comparing ideas to find relations among them, while probable reasoning concerns matters of fact. He considers mathematical reasoning from the relation of ideas category and causal reasoning from the category of matters of fact. He asks us to look at instances of actions where these two types of reasoning are relevant and says that when we do, we will see that reason alone couldnt have moved us. No one thinks that mathematical reasoning by itself is capable of moving us. Suppose you want to stay out of debt. This may move you to calculate how much money comes in and how much goes out, but mathematical reasoning by itself does not move us to do anything. Mathematical reasoning, when it bears on action, is always used in connection with achieving some purpose and thus in connection with causal reasoning.