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Jon Turney asserts in his review (THES, May 12) that he has at least one sentence which is non-contentious as it is based on "an incontrovertible biological fact". Perhaps he is optimistic. A new individual does not begin when "a sperm and an egg unite at the moment of conception". It probably begins when individuation is initiated at gastrulation, some two weeks later in humans. The "moment of conception" is not a clear term either.

Perhaps he means the "moment" of fertilisation. But fertilisation is an extended process, and covers many events. These are baldly described as the sperm and egg uniting. I note with interest that he teaches science communication.

JOHN C. McLACHLAN

School of biological and medical sciences

University of St Andrews