Why did I write this book?

As a lecturer in moral philosophy and in the philosophy of the human person, it has always been important for me to know when a human person begins. In cases of rape it was necessary to know how long after the attack it was morally permissible to attempt to prevent a human embryo originating as a result of violence. This knowledge was crucial in differentiating morally between actions that prevented conception and those whose effect was really abortifacient. It became more pressing for me to be sure of my grounds on this issue once the freezing of human embryos began in some programmes of in vitro fertilization. Community debates in the media on the moral status of the human embryo convinced me of the necessity to inquire further into this question. Living in Melbourne provided the motivation and opportunities to learn the relevant scientific facts of early human embryology for a proper philosophical consideration of the question.

I had always believed, and taught for over 15 years, the commonly held traditional view that a human person begins once the process of fertilization is completed, i.e. once the pronuclei of the sperm and egg mix together. This gives rise to a single-cell human embryo, a zygote, whose genetic individuality and uniqueness remain unchanged during normal development. From that point on, cell divisions and differentiation are programmed for the organization and growth of the same developing human individual already present in the zygote.