Michael Mukasey stands to become the chief law enforcement officer of United States, so his opinion on the legality of various interrogation techniques is more than an abstract or academic concern. Mukasey can say with great certainty that torture is unconstitutional, but that's not so meaningful if you don't quite know what torture is. He's not so sure about waterboarding (which was used in the Spanish Inquisition):

Sen. Whitehouse called Mukasey's answer "purely semantic" and "a massive hedge." I'd also add "absurd." If it's hard for him to make up his mind about whether waterboarding is torture, it should be easy for us to make up our minds about him.