[This article was co-authored by Patrick Lee, Christopher Tollefsen, and Robert P. George.]
In various places we have defended the position that a new human organism, that is, an individual member of the human species, comes to be at fertilization, the union of the spermatozoon and the oocyte. This individual organism, during the ordinary course of embryological development, remains the same individual and does not undergo any further substantial change, unless monozygotic twinning, or some form of chimerism occurs. Recently, in this Journal Jason Morris has challenged our position, claiming that recent findings in reproductive and stem cell biology have falsified our view. He objects to our claim that a discernible substantial change occurs at conception, giving rise to the existence of a new individual of the human species. In addition, he objects to our claim that the embryo is an individual, a unified whole that persists through various changes, and thus something other than a mere aggregate of cells. Morris raises a number of objections to these claims. However, we will show that his arguments overlook key data and confuse biological, metaphysical, and ethical questions. As a result, his attempts to rebut our arguments fail.