A Treatise of Human Nature

Hume published the Treatise in 1739, at twenty-eight, and later complained that it "fell dead-born from the press." It did not. It is the most thorough attempt in modern philosophy to ground all knowledge in experience and to show what happens to metaphysics when you do.

Book I ("Of the Understanding") begins with a simple distinction: impressions are vivid sensory experiences; ideas are their faint copies. All legitimate ideas must trace back to impressions. From this principle Hume dismantles the concept of necessary connection between cause and effect. We observe constant conjunction (one event regularly following another) and feel a habit of expectation, but we never perceive a power compelling the effect. Causation as commonly understood is a projection of the mind onto experience, not a feature read off the world. The same empirical scalpel cuts into personal identity: introspection reveals a bundle of perceptions in constant flux, not a persisting self. Book II ("Of the Passions") maps the mechanics of emotion, including pride, humility, love, and hatred, and argues that reason alone never motivates action. "Reason is, and ought only to be the slave of the passions."

Book III ("Of Morals") extends this into ethics. Moral distinctions are grounded not in reason but in sentiment. Virtue is whatever quality of character earns the approval of a suitably positioned observer. Justice, unlike benevolence, is an "artificial" virtue, arising from convention under conditions of moderate scarcity rather than from natural instinct.

The Treatise set the terms that Kant said woke him from dogmatic slumber. Its arguments about causation, identity, and the limits of reason remain live problems.