A popular response to proponents of embryonic stem cell research and advocates of abortion rights alike—summarized by claims such as “you came from an embryo!” or “you were a fetus once!”—enjoys a rich philosophical pedigree in the arguments of Hare, Marquis, and others. According to such arguments from potentiality, the prenatal human organism is morally valuable because every person's biological history depends on having completed embryonic and fetal stages. In this article I set out the steps of the underlying argument in light of how it has been cast in the philosophical literature and uncover an intriguingly illicit inference.