“Reason is, and ought only to be, the slave of the passions,”
Hume argues that reasoning can inform means and consequences but does not by itself supply the motivating ends that move a person to act.
Source details
A Treatise of Human Nature, Book II, Part III, Section III, “Of the Influencing Motives of the Will,” Project Gutenberg #4705. The frozen catalog retains the opening clause “Reason is, and ought only to be, the slave of the passions,” from a longer sentence. It remains Hume’s historical philosophical thesis rather than contemporary empirical psychology.