The debate sparked off by the Warnock Committee report on human fertilization should benefit from this unusual contribution by Professor O'Donovan. As befits the Regius Professor Moral and Pastoral Theology in the University of Oxford he approaches the topic at a high level of generality, in sharp contrast to the pragmatic and piecemeal procedure often favoured by practising scientists. He modestly claims to make no more than a contribution to the discussion, but his main argument, if accepted, would radically change the thrust of much current practice in this field. * Nature *