The only sanity-preserving way to think about what happened on September 11th is as a 'natural disaster', in which human beings were caught up as tragic agents as well as tragic victims.
Had it been an earthquake, we would not have mourned the less, we would not have been any the less determined to prevent a re-occurrence. But, by now, we would have been energised by our capacity to explain, and on the back of this to make good and to overcome.
Our duty as scientists in response to the present crisis is no different. We must try to explain the shifting plates of human psychology and culture, and show why and how these can erupt in individual acts of such madness. This is not to recommend forgiveness — no one forgives a volcano or a hurricane — but it is to oppose any interest in retribution.
When the perpetrators are brought to trial — normal, less than 'infinitely just' human trial — there will be a single, but obvious, defence: that these pathetic human beings acted 'while the balance of the mind was disturbed'. We must try to understand just what this meant — and means.