DIVERSITY, it seems, is in the eye of the beholder. Conservatives view Judge Clarence Thomas as someone whose background, growing up poor and black in the rural South, distinguishes him from any past nominee to the Supreme Court.Liberals don't deny that, but say the diversity that counts is philosophical. Judge Thomas, they suspect, will be almost indistinguishable from his conservative white colleagues on the court. There's truth in both perspectives. No nominee to the high court has had a life experience that parallels Thomas's. He has known poverty first-hand. But he's also a model of the self-made man. His qualities of courage and drive appeal to Americans of whatever political persuasion. Regarding judicial persuasion, Thomas's record is fuzzier. After only a year on a lower federal bench, he has no trail of rulings on issues like abortion and church-state separation. But his stand against affirmative action, his statements on judicial restraint, his agreement with criticism of the court's 1973 abortion decision - all speak clearly enough of underlying philosophy. If confirmed, he's likely to cement the conservative majority. Is Thomas, as President Bush proclaims, the candidate "best qualified at this time"? That's for political consumption. The nominee's ability and intelligence are undoubted, but in neither his case nor that of David Souter has Mr. Bush opted for heavyweights in constitutional law. These men could grow significantly on the job. Both are young; they'll almost certainly have many decades on the court to hone their thinking. Meanwhile, the terms immediately ahead are likely to be dominated by the thinking of Chief Justice Rehnquist and Associate Justice Scalia, the court's intellectual powerhouses. Thomas will almost surely be confirmed, but his confirmation hearings should be unstinting in their effort to provide a clearer insight into this nominee's vision of American justice.