It turns out that the much-maligned Justice Clarence Thomas has his uses. During oral arguments over affirmative action the other day, he had a question for John Payton, a lawyer for the University of Michigan who was arguing the supposed benefits of racial diversity. What about traditionally black colleges, Thomas asked. "Wouldn't the same argument apply to them?"

I was not present in the Supreme Court that day, so I don't know if Payton paused in mute recognition that if this was chess, he had been checkmated. But then, good lawyer that he no doubt is, he replied that these colleges, although almost entirely black, "do have diverse student bodies."

Well, sorta.

Traditional black colleges and universities do have some nonblacks -- 17.6 percent overall, but below 5 percent in some cases. In fact, if you ask African Americans why they prefer these schools to others, they will cite what amounts to their very lack of diversity. They feel at home there.

I cite the exchange between Payton and Thomas for three reasons. In the first place, they are both black. When one African American sits on the Supreme Court and another represents one of the nation's top universities in a case of maximum importance, then we cannot still be talking about an era when black people were second-class citizens. Second, the face-off between Thomas and Payton represents the virtues and the limitations of what we call diversity. They are both black, both men and yet they differ on this most important of social and legal issues. This clash of ideas -- irrespective of race -- is the sort of diversity American universities ought to promote. Last, we get to what ails affirmative action: Sooner or later the argument for it gets so attenuated it becomes downright silly. The reason is that some justification has to be found to overcome the obvious -- that the constitutional protection of equal treatment under the law is being violated. Someone is being rejected on account of race. It is that simple.

At Michigan, it was Jennifer Gratz. She alleges that she was denied admission because black applicants were favored. Whether she was denied admission for some other reason we may never know. We do know that the university awarded black applicants 20 points (out of 150) on account of race. By comparison, applicants with perfect SAT scores got 12 points. Race mattered -- and mattered greatly. To do the sort of damage Michigan did to Gratz -- she had a 3.8 GPA -- you have to come up with an awfully compelling reason. At first that was easy -- the legacy of slavery and subsequent discrimination. African Americans had to catch up. But the passage of time has weakened that argument, and so advocates of affirmative action have moved on to the supposed virtues of diversity.

In and of itself, diversity is good. But as a new study by some social scientists suggests, its benefits have been oversold. Whatever the case, if diversity is so crucial that it justifies racial discrimination by majority-white institutions, then why isn't it just as important at majority-black colleges? The answer has nothing to do with diversity and everything to do with giving blacks an advantage. This is typical when it comes to defending affirmative action. A worthy societal goal is constantly being defended by well-meaning people who simply will say almost anything in its defense. The first step, almost always, is to deny that an affirmative action program exists. The second is to say it does not entail quotas, and the third is to exalt diversity -- the ultimate justification of the unjustifiable.

The late George C. Wallace -- a race-baiting demagogue if ever there was one -- used to denounce the sort of people who supported busing for the purpose of achieving school desegregation as "pointy-headed intellectuals." Wallace's message resonated because he tapped into the anger some people had that they were paying the price for someone else's sense of social justice. This is somewhat the case now with affirmative action -- but with none of the urgency or justification that Wallace so blithely ignored. It's one thing to overcome segregation. It's something else to use the increasingly distant past to justify an ongoing injustice.

Jennifer Gratz was born in 1978. It's absurd to hold her accountable for slavery or Jim Crow, and unfair to handicap her because of her race. She played by the rules we're all supposed to live by, and she wanted nothing more than to be judged on her merits. Instead, she got judged -- critically -- on her race.

That's wrong -- constitutionally, I think, morally I am sure.