What did David Hume mean when he called reason the slave of the passions?

When the Scottish philosopher David Hume famously described reason as the "slave of the passions," he was arguing that reason alone is incapable of motivating human action or determining moral ends.12

For Hume, reason is a tool for calculation; it can help people identify the most effective means to achieve their goals, but it cannot generate those goals itself. Because morality is essentially practical—meaning it must incline individuals to act—Hume concluded that moral judgments cannot be derived from reason alone. Instead, action is always driven by a prior "passion," such as a desire, want, or feeling.12

In this view, reason is subservient to these underlying desires because it can only function once a passion has already established an objective. Consequently, Hume maintained that moral distinctions are not matters of rational truth but are instead rooted in human sentiment and emotion.12