Professor Frey expresses surprise at my assertion that, ultimately, nothing is provable in ethics. What about Pol Pot’s atrocities, he asks—surely we can all condemn them? Let us take a more recent example, the Beslan school massacre. The terrorists appear to have weighed against the unquestionably serious harm to the children (and adults) the boost to their cause which they judged the attendant publicity would achieve. I may believe that even to attempt such a utilitarian assessment is obscene, and most would no doubt agree. I cannot, however, prove that this is the wrong approach. A few people, at least, take a different view.

The example shows the problem with utilitarian assessments where the harm is both certain and direct, the victims and beneficiaries are wholly distinct and the victims have no say in the assessment—precisely the pattern with vivisection.

I do not, of course, advocate an ethical free for all. As I …