James Rachels, answered from the texts and cited to the page.
Morality is, at the very least, the effort to guide one's conduct by reason — to do what there are the best reasons for doing — while giving equal weight to the interests of each individual affected by one's action.1 That formulation has two parts, and both matter. The first is the commitment to reason: not sentiment, not habit, not the unexamined customs of one's community, but the careful sifting of facts and their implications.
The conscientious moral agent accepts principles only after scrutinizing them to make sure they are justified, and is willing to revise beliefs when the argument requires it.2 The second is impartiality. To be impartial is to treat everyone alike, to give no one's interests automatic priority, and — crucially — to refuse to treat members of particular groups as inferior.3
Racism fails this test not because it offends some vague sentiment but because the racist cannot produce a good reason for the differential treatment he insists upon. When there is no good reason to treat people differently, doing so is unacceptably arbitrary.4 Notice that impartiality does not mean treating all situations identically regardless of context.
If someone is casting a film about Fred Shuttlesworth, there is a perfectly good reason to prefer an African-American actor for the role; that is not discrimination.5 The requirement is against arbitrary differential treatment, not against all differential treatment. The minimum conception is a floor, not a ceiling. It does not settle every dispute between utilitarians, Kantians, virtue ethicists, and the rest.
What it does is mark out the space within which serious moral reasoning takes place: reason, applied impartially. Any theory that falls below that floor — that licenses arbitrary favoritism or refuses to answer to reasons at all — has already disqualified itself from the conversation.
Morality is, at the very least, the effort to guide one's conduct by reason—that is, to do what there are the best reasons for doing—while giving equal weight to the interests of each individual affected by one's action.The Elements of Moral Philosophy, pp. 28–29
The conscientious moral agent is someone who sifts facts carefully and examines their implications; who cares about everyone's interests impartially; who accepts principles of conduct only after scrutinizing them to make sure they are justified; who will 'listen to reason' even when it means revising their beliefs.The Elements of Moral Philosophy, pp. 28–29
To be impartial is to treat everyone alike; no one gets special treatment... Impartiality also requires that we not treat the members of particular groups as inferior. Thus it condemns forms of discrimination like sexism and racism.The Elements of Moral Philosophy, pp. 28–29
if there is no good reason to treat people differently, then to do so is unacceptably arbitrary; it is discrimination.The Elements of Moral Philosophy, pp. 28–29
Suppose someone were making a movie about Fred Shuttlesworth (1922-2011), the heroic African-American civil rights leader. This person would have a good reason not to cast Chris Pratt in the starring role—namely, that Pratt is white. Such a decision would not be arbitrary or objectionable; it would not be discrimination.The Elements of Moral Philosophy, pp. 28–29