Ethics has developed as people have reflected on the intentions and consequences of their acts. From this reflection on the nature of human behavior, theories of conscience have developed, giving direction to much ethical thinking. Intuitionists (Ralph Cudworth, Samuel Clarke), moral-sense theorists (the 3d earl of Shaftesbury, Francis Hutcheson), and sentimentalists (J. J. Rousseau, Pierre-Simon Ballanche) postulated an innate moral sense, which serves as the ground of ethical decision. Empiricists (John Locke, Claude Helvétius, John Stuart Mill) deny any such innate principle and consider conscience a power of discrimination acquired by experience. In the one case conscience is the originator of moral behavior, and in the other it is the result of moralizing. Between these extremes there have been many compromises.

Another major difference in the approach to ethical problems revolves around the question of absolute good as opposed to relative good. Throughout the history of philosophy thinkers have sought an absolute criterion of ethics. Frequently moral codes have been based on religious absolutes. Immanuel Kant, in his categorical imperative, attempted to establish an ethical criterion independent of theological considerations. Rationalists (Plato, Baruch Spinoza, Josiah Royce) founded their ethics on a metaphysics.

All varying methods of building an ethical system pose the question of the degree to which morality is authoritative (i.e., imposed by a power outside the individual). If the criterion of morality is the welfare of the state (G. W. Hegel), the state is supreme arbiter. If the authority is a religion, then that religion is the ethical teacher. Hedonism, which equates the good with pleasure in its various forms, finds its ethical criterion either in the good of the individual or the good of the group. An egoistic hedonism (Aristippus, Epicurus, Julien de La Mettrie, Thomas Hobbes) views the good of the individual as the ultimate consideration. A universalistic hedonism, such as utilitarianism (Jeremy Bentham, James Mill), finds the ethical criterion in the greatest good for the greatest number.

Among ethical theories debated in the first half of the 20th cent. were instrumentalism (John Dewey), for which morality lies within the individual and is relative to the individual's experience; emotivism (Sir Alfred J. Ayer), wherein ethical considerations are merely expressions of the subjective desires of the individual; and intuitionism (G. E. Moore), which postulates an immediate awareness of the morally good. Agreeing with Moore that the morally good is directly apprehended through intuition, deontological intuitionists (H. A. Prichard, W. D. Ross) went on to distinguish between good and right and to argue that moral obligations are intrinsically compelling whether or not their fulfillment results in some greater good.

Important ethical theories since the mid-20th cent. have included the prescriptivism of R. M. Hare, who has compared moral precepts to commands, a crucial difference between them being that moral precepts can be universally applied. In his arguments for virtue ethics, Alasdair C. MacIntyre has cautioned against unbridled individualism and advocated correctives drawn from Aristotle's discussion of moral virtue as the mean between extremes. Thomas Nagel has held that, in moral decision making, reason supersedes desire, so that it becomes rational to choose altruism over a narrowly defined self-interest. See also bioethics.