A Treatise of Human Nature

A Treatise of Human Nature comprises three sections: Of the Understanding, Of the Passions, and Of Morals. Of the Understanding looks at the nature of ideas, the ideas of space and time, concepts of knowledge and probability, skepticism, the soul, and personal identity. Of the Passions covers pride and humility, love and hatred, and the will and direct passions. Of Morals examines general concepts of virtue and vice, justice, and specific virtues.

In Of the Understanding, Hume argues that complex ideas are formed from simple ideas, which are formed from impressions based on direct experience. Consequently, ideas are not fundamentally different from impressions. Hume divides impressions into two categories: original and secondary. Original refers to internal impressions that reach us through our senses from physical sources. Secondary impressions are based on those original impressions. Hume argues that experience must be the basis for “matters of fact”; they cannot be approached by reasoning. If we have no direct experience of a concept (such as the size of the universe), that concept is not meaningful. For this reason, Hume argued that since we have no direct experience of God or the soul we cannot have any meaningful knowledge of them. Hume introduces three philosophical concepts: the microscope, the razor, and the fork. The microscope is the idea that in order to understand complex ideas, the philosopher has to break them down into simple ideas that can be analyzed. The razor is the idea that in order for a term to have meaning, it must be related to an idea that can be broken down into simple, concrete ideas. If the term fails that test, it is meaningless. The fork is the idea that truths can be divided into two kinds. The first kind of truth refers to things that arise out of relationships between ideas—such as math. The second kind of truth refers to matters of fact—things that exist in the world.

In Of the Passions, Hume returns to his distinction between original and secondary impressions, arguing that the passions are in the latter. Hume distinguishes between direct and indirect passions. Within those categories, Hume separates the cause and object of various passions. Hume argues that passions are related not to reason but to action. Human action is motivated not by reason but by passions. As such, morality cannot be based on reason.

In Of Morals, Hume builds on the previous two books, arguing that the two categories of morals, vice and virtue, are based on impressions rather than ideas: vice on the impression of pain, and virtue on the impression of pleasure. He argues that moral impressions come only from human actions and are valid only insofar as they apply socially. Consequently, sympathy, argues Hume, is the basis for moral obligation. Morality, says Hume, is not a direct matter of fact, based on direct experience of the physical world. Rather, it is based in the passions, arising from secondary impressions. Though reason can help understand morality it cannot be the basis for it.

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