WASHINGTON -- Harvard Law Professor Laurence H. Tribe initially praised John G. Roberts Jr., President Bush's nominee to the Supreme Court. In an interview with The New York Times July 21, Tribe said his former pupil was "quite deeply immersed in the law, and he loves it. He believes in it as a discipline and pursues it in principle and not by way of politics."

But on Saturday night, in a speechat the convention of the American Constitution Society, Tribe was much more critical. The dean of liberal legal scholars apparently had been scared straight (from the anti-Roberts point of view) by the release of memos from the nominee's days as a young and right-leaning lawyer in the Reagan administration.

For example, as a lawyer in the White House counsel's office, Roberts took shots at a supposedly activist Supreme Court -- one that invaded the turf of other branches of government. Roberts took this view, Tribe noted in his speech, after the court had moved away from the aggressive defense of equality and individual rights that characterized the era of Chief Justice Earl Warren.

Elsewhere in his speech, however, Tribe mocked Roberts' assurances to a senator that he wouldn't follow an activist philosophy to the court.

Tribe, who believes the Senate should closely question Supreme Court nominees about their views, said a fuller explanation was in order -- though Tribe seemed resigned to the fact that the nimble Roberts wouldn't be any more forthcoming in his hearings than he has been in his courtesy calls.

At first glance, these two criticisms of Roberts -- that he criticized judicial activism as a young lawyer and that he might be an activist himself on the court -- might seem contradictory.

If an activist Supreme Court is a good idea, then Roberts was wrong as a young lawyer and would be right now if he decided to be an activist justice, On the other hand, if judicial activism is bad, then the young Roberts was right and Tribe should hope he's sincere in his protestations about not being an activist.

In fairness to Tribe and others who worry that Roberts might be an "activist" justice, the issue of "judicial activism" needs to be unpacked (as literary critics say) and it would be educational if Roberts at his confirmation hearings did some of the unpacking himself.

Democrats and liberals recently have tried to turn the activism issue to their advantage by pointing out that it is Republican-appointed conservatives on the court who are the real judicial activists, invalidating important acts of Congress.

"They have struck down parts of the Violence Against Women Act, environmental acts, child-safety legislation," Democratic Sen. Patrick Leahy of Vermont complained last week. Leahy warned he would vote against Roberts if he seemed likely to pursue an "activist" philosophy on the bench. Other liberals have taken up that argument.

Never mind that liberals were happy to say goodbye to some of the laws the Rehnquist court has struck down, such as a federal law against burning the American flag as a protest or an attempt by Congress to reverse the Miranda decision on the rights of criminal suspects.

In general, Leahy is correct. Much of this court's activism has taken the form of striking down liberal legislative acts. In a throwback to Franklin D. Roosevelt's critique of the Supreme Court of the 1930s, liberals say these decisions are exercises in conservative ideology, not constitutional interpretation.

But if it is wrong for the court to be activist in striking down liberal acts of Congress, why isn't it wrong for a liberal court to strike down state laws against abortion, or same-sex sodomy or racially integrated public schools? Does it all depend on whose policy ox is being gored? As senators on the Judiciary Committee and Judge Roberts prepare for his confirmation hearings, I would suggest some not very light summer reading: Thomas M. Keck's book "The Most Activist Supreme Court in History: The Road to Modern Judicial Conservatism."

Keck, a political scientist at Syracuse University, supports the complaint by liberal Democrats that the Rehnquist court is hyper-activist. But liberals won't be pleased to read that some of those activist decisions are rulings they like, such as Lawrence v. Texas, the sodomy decision hailed as a gay-rights landmark.

Keck argues that there is no one on the present Supreme Court who agrees with the late Justice Felix Frankfurter, who believed in across-the-board judicial deference to elected officials. Three justices -- Rehnquist, Thomas, Scalia -- believe in conservative activism but not liberal activism. Four -- Stevens, Souter, Ginsburg and Breyer -- believe in liberal activism but not conservative activism. And two -- swing voters Kennedy and O'Connor -- believe in both.

The defining feature of what Keck calls the O'Connor court "has been the success of conservatives in expanding judicial power and exercising that power on behalf of conservative ends." But he sees a cloud in this silver lining for conservatives: "The nation's ever-firmer commitment to rights-based activism, in turn, has undermined the continued calls for judicial restraint in contests such as abortion and gay rights."

It's unlikely that conservatives and liberals will reach some grand consensus on the role of the judiciary that is principled and consistent.

One person's "activism" will often be another person's vindication of constitutional values against a benighted political majority. But Keck's ideas are worth talking about, and the Roberts confirmation hearings could be the forum for such a conversation.

Perhaps Leahy could begin by providing a list of Supreme Court decisions he considers activist and asking Judge Roberts come up with his own. Even if Roberts balks -- or stops the clock at the Dred Scott decision -- the discussion will have advanced beyond the current generalities.