In this Wall Street Journal column, Jan Crawford Greenburg focuses on one of the findings of her new book on the Supreme Court's recent history: that Clarence Thomas is not just a "lackey" of Antonin Scalia's.

In my view, this is actually one of Greenburg's less original revelations. Other writers, such as Thomas biographer Andrew Peyton Thomas (no relation to the Justice), and law professor Scott Gerber have already documented the considerable divergences between Thomas' approach to constitutional law and Scalia's.

Moreover, the two justices' published opinions reveal important differences even aside from the inside sources tapped by Greenburg, Peyton Thomas, and Gerber. They show that Scalia and Thomas diverged on a number of major constitutional issues including censorship of on-line pornography, federalism (especially in Gonzales v. Raich), the line-item veto, the rights of Guantanamo detainees (in Hamdi v. Rumsfeld), and the Public Use Clause (in Kelo v. City of New London, where both voted the same way, but Thomas wrote a separate dissent advocating a much narrower definition of "public use" than that endorsed in the principal dissent by Justice O'Connor which Scalia signed on to). Of course, Scalia and Thomas agree on many more issues than they disagree on. But that is not surprising for two generally conservative justices. The Court's liberal justices agree among themselves with roughly equal frequency.

The systematic disagreements between Thomas and Scalia, to my mind, stem from three principle sources: Thomas' greater commitment to originalism in cases where the original meaning clashes with precedent or modern policy preferences (evident in the federalism cases, especially Raich); Thomas' libertarian streak, which sometimes clashes with Scalia's social conservatism (evident in the First Amendment cases where they disagree; and perhaps also in Kelo); and Thomas' commitment to a broader view of executive power than Scalia is willing to support (as in the Guantanamo cases, where Thomas is the only justice to fully endorse the Bush Administration's sweeping claims of wartime executive power).