No one can say when exactly a foetus becomes a person

THE fundamental objection that Prof William Reville (Letters, November 1) has to human embryonic stem cell research (HESCR) is based on his belief that at fertilisation when sperm and ovum join, a full human being comes into existence.

By this I understand him to say he believes the single cell, called the zygote, is a full human person.

If this is granted, then of course there can be no argument — the zygote, as a person, is entitled to the same moral status and the same full protection of the law as a born baby.