James Rachels, answered from the texts and cited to the page.
Reason in ethics is not a separate faculty that operates independently of our moral experience — it is the requirement that we act on the best available grounds, giving equal weight to the interests of each person affected by what we do. Morality, at its minimum, is the effort to guide conduct by reason while treating no one's interests as automatically more important than anyone else's.1
Impartiality follows directly from this. To be impartial is to treat everyone alike unless there is a relevant difference that justifies treating them differently.2 The racist who insists whites should hold all the best jobs cannot point to any such difference — whites are not inherently brighter, harder working, or more deserving of benefit than others — and so the unequal treatment is arbitrary, which is precisely what the requirement of reason forbids.3
Racism is, at bottom, an offense against reason, not merely against sentiment. The same logic applies to nationalism, sexism, and classism: each divides humanity into the morally favored and the disfavored without being able to supply the relevant difference that would justify the division.4 That said, impartiality does not mean treating everyone identically in every circumstance.
Once we admit that relevant differences can justify differential treatment — and we must admit this, since imprisoning a convicted criminal while leaving innocent citizens free is plainly not a failure of impartiality — the question becomes which differences genuinely count.5 A filmmaker casting a biographical film about Fred Shuttlesworth has good reason not to cast a white actor in the role; that is not discrimination.6
The hard philosophical work lies in distinguishing differences that genuinely justify unequal treatment from those that are mere hand-waving. One persistent challenge comes from Hume's point that reason alone cannot justify our ultimate starting assumptions — a utilitarian assumes that maximizing welfare is what matters, someone else begins differently, and reason by itself cannot adjudicate between them.7
MacIntyre presses this further, arguing that standards of rational justification are themselves tradition-bound, so that the search for norms binding on all people regardless of historical tradition is an Enlightenment illusion.8 These are serious objections. But even granting that reason alone cannot determine which ultimate principles to accept, it does not follow that the choice between principles is arbitrary — there are constraints on what principles we can coherently choose, and those constraints do real philosophical work.9
There is also a pleasing convergence worth noting. Reason requires impartiality; social living requires adherence to rules that serve everyone's interests fairly applied; and we are, as a matter of evolutionary fact, social creatures who care about one another's welfare. Psychological egoism — the claim that we can only ever care about ourselves — is false, and its falsity means reason's demand for impartiality is not an impossible standard.10 Morality is not only possible for us; to a large extent, it is natural for us.
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
We can say that impartiality requires us to treat people in the same way only when there are no relevant differences between them.Can Ethics Provide Answers, pp. 232–233
Is there something about white people that makes them better fitted for the highest-paying and most prestigious jobs? Are they inherently brighter or harder working?... In each case, the answer is no; and 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
Racism is an offense against morality because it is an offense against reason. Similar remarks apply to other doctrines that divide humanity into the morally favored and disfavored, such as nationalism, sexism, and classism.The Elements of Moral Philosophy, p. 196
it is not a failure of impartiality to imprison a convicted criminal while innocent citizens go free, because there is a relevant difference between them (one has committed a crime; the others have not) to which we can appeal to justify the difference in treatment.Can Ethics Provide Answers, pp. 232–233
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
We can never justify all our assumptions; reasoning must begin somewhere. A utilitarian might begin by assuming that what is important is maximizing welfare. Someone else, with a different cast of mind, might make a different assumption. But reason alone cannot justify the choice of one starting point over another.The Legacy of Socrates, p. 232
MacIntyre argues that 'rationality' has meaning only within a historical tradition... the standards of rational thinking differ from tradition to tradition, and so we cannot speak of 'what reason requires' in any universal sense.The Legacy of Socrates, p. 232
even if 'reason alone' cannot determine what ultimate principles we should accept, this does not mean the choice must be arbitrary. There constraints what principles choose, and theseThe Legacy of Socrates, p. 232
Psychological Egoism is not true; it presents a false picture of human nature and the human condition. We have evolved as social creatures, living together in groups, wanting one another's company, needing one another's cooperation, and caring about one another's welfare. So there is a pleasing 'fit' between (a) what reason requires, namely, impartiality; (b) the requirements of social living... and (c) our natural inclination to care about others.The Elements of Moral Philosophy, p. 196