On What Matters by Derek Parfit Oxford University Press, Volume 1: 540 pp., $35.00; Volume 2: 825 pp., $35.00

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Philosophers have long sought to formulate a theory that explains the purposes of commonsense moral rules and provides principles enabling us to resolve the frequent moral dilemmas we encounter. Thomas Hobbes wrote that familiar moral rules are not relative to one culture or another but are “articles of peace,” necessary to civilized social life. It is in everyone’s rational self-interest to obey these rules; the grim alternative is a “state of war.” Immanuel Kant said that we have an unconditional duty to obey morality regardless of our desires and self- interests. His second “categorical imperative” says that we ought never treat others “merely as means,” but always as “ends in themselves.” To do so, we should follow a general principle that we believe everyone should follow in circumstances like our own.

Kant held that his imperative justifies our commonsense duties to each other and provides a more fine-grained method of reasoning about what we ought to do when ordinary moral rules do not adequately address the complexities of life. For example, under what circumstances is it permissible to break a promise or deceive someone? To save innocent life or prevent great harm, surely, but not to benefit ourselves in minor ways; the hard cases lie in between.

In The Methods of Ethics (1874), among the greatest works in moral philosophy since Kant, the British philosopher Henry Sidgwick countered that the rules of commonsense morality coincide with utilitarianism. The “principle of utility” says that our actions are right and our laws are just to the degree that they promote the greatest sum of “utility,” or happiness, in the world. Happiness, Sidgwick says, is basically pleasurable experiences.

Contemporary utilitarians often identify happiness (now called “welfare” or “well-being”) with satisfaction of preferences, or of rationally informed desires. Until John Rawls’s influential social contract theory, expounded in A Theory of Justice (1971), utilitarianism remained the predominant moral theory in Anglo-American philosophy for over two hundred years. Utilitarianism remains highly influential among economists, in business and law schools, and in public policy institutes. In each it is common to hear arguments that a law or practice is justified because it improves overall well-being.

Utilitarianism is the most prominent example of a family of positions called “consequentialism.” These positions hold that actions, laws, or other conventions are right to the degree that they produce the best consequences, effectively “maximizing” the good. Many consequentialists today consider utilitarian general happiness only one of the good consequences that right conduct ought to promote. Some say that equalizing the distribution of happiness is also important. Some consequentialists endorse, as among the “intrinsic goods” that ought to be promoted, goods such as knowledge, creativity, aesthetic appreciation, love and friendship, or individual freedom.

Joining consequentialism and Kantianism is a third major position in contemporary moral philosophy, Harvard philosopher T.M. Scanlon’s “contractualism,” which reflects…