A Treatise of Human Nature

Overview

The Treatise is six hundred and twenty pages in three books, published anonymously in 1739 and 1740, and written mostly in France at La Flèche while Hume was in his early twenties. He later said it fell dead-born from the press, which is not quite true but close enough. It was reviewed, briefly and badly, and largely ignored, and it cost him a professorship when the parts about religion were used against him.

The subtitle tells you the ambition: an attempt to introduce the experimental method of reasoning into moral subjects. Hume wants to do for the mind what Newton did for matter, which means finding a few principles of association from which the phenomena follow, and refusing to speculate about ultimate causes. His word for the whole enterprise is the science of man, and he argues that every other science depends on it, since all of them are conducted by human beings using human faculties.

A practical warning. Hume disowned the form, though not the content. He rewrote Book I as the Enquiry Concerning Human Understanding in 1748 and Book III as the Enquiry Concerning the Principles of Morals in 1751, asked to be judged by those, and called the Treatise a juvenile work. Most philosophers now think the Treatise is the greater book and the Enquiries are the better introduction. If you are reading Hume for the first time, start with the first Enquiry, which is a quarter the length and covers most of the famous arguments.

The argument

Book I: Of the Understanding. Everything begins with a distinction. Perceptions come in two kinds: impressions, which are lively, and ideas, which are the faint copies of impressions that we have in thinking and remembering. Every simple idea derives from a simple impression. This gives Hume a test he uses throughout: when a word seems to mean something, ask from what impression its idea is derived, and if there is none, the word is empty. He notes one counter-example himself, a shade of blue you have never seen but could imagine from its neighbours, and moves on with a candour that has irritated commentators ever since.

Ideas are connected by three principles of association, resemblance, contiguity and cause and effect, which are his gravitation.

Then the argument about causation, which is the most important in the book. We say a cause necessitates its effect. Look for the impression of that necessity. What you actually observe is that the two events are next to each other, that one comes first, and that this pairing has happened repeatedly. Constant conjunction is all the evidence there is. The necessity is not in the objects; it is a felt determination of the mind to pass from one idea to the other, a habit produced by repetition and then projected onto the world.

From this comes the problem of induction. Why expect the future to resemble the past? Not by demonstration, since a change in the course of nature implies no contradiction. Not by experience, since any argument from experience already assumes that nature is uniform, which is the thing to be shown. There is no non-circular justification. What carries us is custom, which Hume calls the great guide of human life, and he is entirely serious that this is not a defect to be repaired.

Part 4 turns the corrosive on Hume himself. Reason undermines itself when applied to its own reliability. The senses cannot establish the existence of bodies; belief in continuing objects is a fiction produced by the imagination out of the constancy of our impressions. And the self: look inward and you catch a perception, never a perceiver, so the mind is a bundle of perceptions in perpetual flux, and the theatre metaphor must be resisted because there is no stage.

He ends Book I with a remarkable passage of despair, describing himself as a monster cast out of company by his own conclusions, unable to find a footing anywhere, and then rescued by dinner, backgammon and conversation with friends, after which the arguments look ridiculous. That is not a joke. It is his account of the relation between philosophy and life.

Then, in the Appendix published with Book III, he returns to personal identity and admits that his own account will not do, that he cannot explain what unites the successive perceptions, and that on this head all his hopes vanish.

Book II: Of the Passions. The least read and the hinge of the whole system. Hume gives a mechanical account of pride, humility, love and hatred, each produced by a double relation between an impression and an idea. He introduces sympathy, the process by which we convert an idea of another person’s feeling into something we feel ourselves, which he needs for the moral theory.

The famous section is on the influencing motives of the will. Reason, being concerned with relations of ideas and matters of fact, can never by itself produce or prevent an action; it can only tell you how to get what you already want. So the notorious sentence, that reason is and ought only to be the slave of the passions, is not a celebration of unreason. It is a claim about what reasoning can do, followed by the deliberately shocking observation that reason cannot convict a preference of error, however monstrous.

Book III: Of Morals. Moral distinctions are not derived from reason, because reason alone cannot move us and moral judgements do. They come from sentiment: an action is virtuous when contemplating it produces a particular kind of pleasure in a spectator taking a general point of view.

Then the paragraph that has outlived everything else. Hume observes that writers on morality proceed by ordinary reasoning about what is and then, without warning, start writing about what ought to be, and that this transition needs explaining, since a new relation cannot be deduced from others entirely different. That is the is-ought gap.

The rest of Book III distinguishes natural virtues, which please directly, from artificial ones, of which justice is the type. Justice is a human contrivance, not an instinct. It arises because human beings are moderately selfish and goods are moderately scarce, and it consists in a convention establishing the stability of possession, transfer by consent, and the keeping of promises. Hume is explicit that in conditions of abundance or of universal benevolence, justice would be pointless.

How to read it

Read the first Enquiry instead, if this is your first Hume, then come back.

If you read the Treatise, read Book I part 3 sections 2 to 8 on causation and induction, Book I part 4 sections 5 to 7 on the self and the crisis, Book II part 3 section 3 on reason and the passions, and Book III part 1. That is the heart and it is perhaps a hundred and twenty pages.

Read the Appendix on personal identity immediately after Book I part 4 section 6, rather than where it is printed. Hume retracting his own argument is one of the most honest things any philosopher has done and it changes how the earlier section reads.

Keep the copy principle in hand as a tool. Almost every destructive argument in the book is an application of one question: from what impression is that idea derived?

Do not mistake the scepticism for nihilism. Hume’s conclusion is not that we should stop believing in causes, objects or selves, which he says we cannot do. It is that these beliefs have a different source from the one philosophy assumed, and that nature has not left something so important to our reasoning.

Questions it raises

• If there is no impression of necessary connection, what are the sciences doing when they state laws?
• Hume says induction has no justification and that we cannot stop relying on it. Is that a solution, a dissolution, or a shrug?
• The copy principle rules out ideas with no impression behind them. Does it survive his own missing shade of blue?
• The self is a bundle, and then Hume admits he cannot say what binds it. Has anyone answered him?
• If reason cannot convict a preference of error, can any moral argument ever be more than persuasion?
• Justice is called an artificial virtue that arises from scarcity and limited generosity. Does that make it less binding, and did Hume think it did?