The pro-life argument from substantial identity: a defence.

This article defends the following argument: what makes you and I valuable so that it is wrong to kill us now is what we are (essentially). But we are essentially physical organisms, who, embryology reveals, came to be at conception/fertilisation. I reply to the objection to this argument (as found in Dean Stretton, Judith Thomson, and Jeffrey Reiman), which holds that we came to be at one time, but became valuable as a subject of rights only some time later, in virtue of an acquired characteristic. I argue against this position that the difference between a basic, natural capacity and some degree of development of such a capacity is a mere difference in degree, that this position logically implies the denial of equal personal dignity, and that the selection of the required degree of development of a capacity is necessarily arbitrary.