Men often act knowingly against their interest.

The line encapsulates a central Humean theme: human conduct is not reliably governed by cool calculation of self-interest. Even when people clearly recognize what would benefit them, passions, habits, social pressures, and short-term impulses can override that recognition. In Hume’s moral psychology, reason is limited in its motivational power; it can inform us about means and consequences, but it does not by itself move the will. The quote also gestures toward Hume’s skepticism about purely rationalist accounts of ethics and politics, implying that institutions and moral education must account for predictable irrationality and weakness of will rather than assuming consistent prudence.