In particular, he does a great job of debunking the "ticking time-bomb" scenarios that is most often used to justify removing policies that prohibit torture. And he does so on two grounds: 1) The scenario is completely unrealistic, and therefore inappropriate as a basis for policy, and 2) Practically speaking, no policy would apply in such a scenario:
Just because you can construct a hypothetical scenario were shooting a girl in the head is the "right" thing to do, that doesn't mean that we should do away with the legal prohibition against murder. When it comes to acts that are sufficiently bad--such as murder and torture--you need categorical rules.I recommend reading the whole piece. It's really very clear and well-reasoned. Policies on extraordinary conduct require an extraordinary amount of thought and clarity. Making emotionally-charged decisions based on irrational scenarios is a prescription for bad policy (see USA PATRIOT Act, for example).
In a true ticking bomb scenario (which I'm convinced is like saying "when you meet a real unicorn"), people will do what they think they have to do, regardless of what the law says. And in that kind of extraordinary situation, no one would be prosecuted for resorting to extreme, even illegal tactics.
But you can't let highly unlikely hypothetical scenarios dictate policy. Regardless of whether there are conceivable situations where torture could be justified, it has to remain illegal.