"Men often act knowingly against their interest"

Hume’s aphorism points to a rift between judgment and conduct. People can foresee harm yet proceed, not because reason fails to compute, but because motivation is anchored in passion. Reason, for Hume, discovers means and weighs probabilities; it does not supply ends. When a vivid, violent passion, anger, lust, fear, arises, it eclipses the calmer, more distant interest in health, wealth, or reputation. The agent “knows,” but the knowledge is cool; the passion is hot.

Social passions deepen the paradox. Love of esteem, desire to punish wrongs, pride, and loyalty can prompt self-sacrifice or self-sabotage. A person may risk livelihood to save face, duel for honor, or blow the whistle at personal cost. From a narrow economic calculus this looks perverse; from the standpoint of sentiments and identity, it is coherent, even necessary. What counts as “interest” is plural: immediate pleasure, long-term welfare, moral integrity, and social standing often pull in different directions. Because the mind discounts the future, present impressions strike more forcefully than absent goods. Habit, imagination, and the salience of temptations nudge us toward courses we ourselves judge unwise.

Modern findings echo the insight: present bias, hyperbolic discounting, akrasia, costly punishment, and commitment signaling all reveal agents who act knowingly against material welfare. Knowledge alone rarely overrules passion. Hence Hume’s emphasis on character and institutions: educate the sentiments so that virtue is felt as agreeable and vice as uneasy; shape customs that reward prudence; deploy commitment devices that pit passion against passion, oaths, reputational stakes, precommitments, deadlines.

The word “often” matters. This tendency is common, not aberrant. Human beings are not calculators but creatures of sentiment whose interests are layered and unstable. Recognizing this duality fosters humility in moral judgment and realism in politics: do not presume constant prudence; design environments where the easiest choice aligns with long-term good. When calm reflection prevails, knowledge and interest can coincide; when passions surge, people will sometimes choose otherwise, with their eyes open.