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.
David Hume (1711–1776) was born in Berwickshire, near Edinburgh, in Scotland. Hume was a philosopher, historian, economist, and essayist. He attended the University of Edinburgh from the age of 11 but left at 15 to pursue private study. His skepticism concerning religion kept him from getting the Chair of Ethics and Pnuematical Philosophy at the University of Edinburgh. When he failed to get the position, he accompanied his cousin as a secretary on a military mission against the French in Canada. After his trip to Canada, Hume travelled with his cousin to Vienna and Turin. He wrote at least one important philosophical treatise during this trip. When he returned to Scotland he accepted a position as a librarian and completed the six-volume History of England, which became a best seller. Hume lived in Paris as secretary to the British ambassador to France for three years. A fleeing Jean-Jacques Rousseau accompanied Hume on his return trip to England. Hume lived in London for a year, serving as under-secretary of state. Returning to Edinburgh, he built a house where he remained for the rest of his life.
Hume’s empiricist philosophy centered on his assertion that the science of man is the basis for all other sciences. In other words, one must understand how the human mind works in order to properly understand other sciences. Hume believed that there was no constant, permanent self. Rather, the self is always the sum of one’s sensations and reflections. Knowledge, likewise, is derived from sensations and reflections on those sensations. Consequently, propositions about objects are semantically equivalent to propositions about one’s experiences. While we can have belief in something that is not directly observable, we cannot have knowledge about that thing. Hume taught that “cause” and “effect” were qualities of human perception, not necessarily of the object itself. For example, we see ball A strike ball B; following that, ball B moves. Hume argued that while we perceive ball A to have caused the effect of ball B moving, those qualities might not exist in the balls themselves. The habit of seeing a ball strike another ball, followed by the movement of the second ball, leads us to perceive that ball A caused ball B to move. Hume wasn’t saying that ball A didn’t cause ball B to move; just that we cannot empirically observe the mechanism for the movement, and thus we cannot have knowledge of it.