Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.

Hume formulates this claim in the moral psychology of his early masterpiece, the Treatise, written in the 1730s and published in 1739–40. In Book II (“Of the Passions”) and Book III (“Of Morals”), he argues against rationalist ethics (associated with thinkers like Samuel Clarke) that treat moral judgment and motivation as products of reason alone. For Hume, reason discovers relations of ideas and matters of fact, but it does not by itself generate ends or move the will. The line appears amid his discussion of how action is explained: passions and sentiments supply the motivating force, while reason’s role is instrumental—informing us of facts and causal means to satisfy our desires.

The remark encapsulates Hume’s “motivational internalism”: what ultimately moves us to act is not abstract reasoning but desire, aversion, and sentiment. Reason can correct our beliefs (e.g., about what will achieve an end) and can reveal inconsistencies, but it cannot by itself supply the end to be pursued. Calling reason the “slave” of the passions is deliberately provocative: it denies that rational reflection is the sovereign source of morality or choice. The significance is twofold: it underwrites Hume’s sentimentalist ethics (moral approval arises from feeling) and anticipates later debates in philosophy and psychology about whether practical reason can motivate independently of desire.