People have passionately debated this deceptively simple question down through history. For some people, the answer is easy: they reply, “at fertilisation”. For example, this is the official view of the Roman Catholic Church and many conservative protestant Christians.
It is reinforced by the belief that human life is distinguished from that of animals because humans have a soul, making them uniquely spiritual beings. But at what stage does this spiritual element actually arise? For those with no religious faith, the same question might be, at what stage does the embryo or foetus become a person?