In reality, human development unfolds along a mostly seamless arc, not as a series of discrete graduations. The life of the human organism starts in a reasonably well-understood and biologically observable way and then progresses until it reaches its biological conclusion. There is not usually much debate about when a body is dead. The fact that there are occasional ambiguities and “corner cases” does not somehow leave us unable to see what is in front of our faces.

They had less sophisticated tools of biological analysis in the 19th century, but few of those who held property in slaves or who justified such bondage denied that slaves were human beings — what they argued was that slaves were a less worthy form of human being and as such did not deserve to be recognized as persons under the law. (Contrary to what many people seem to think, in the infamous “three-fifths compromise,” it was the slavers who wanted slaves counted as full people and the abolitionists who did not want them counted at all — the planters had no intention of permitting slaves to vote, but they wanted them counted for congressional-apportionment purposes.) As with the case of abortion advocates who would deny the fact of the human character of the unborn, those who justified slavery had explicit and implicit theologies they could muster in the defense of their case, a whole metaphysical battery they could fire at their abolitionist opponents. They could also point to a long-standing legal order supporting their claims. The abolitionists did not have the law on their side, but they had the facts on their side, and the law eventually was made to account for the facts — for reality.

The question of “fetal personhood” cannot be sorted out as though it were exclusively, or even primarily, a legal question. What is in the womb — what is killed in an abortion — is what it is, and the law does not have the power to change that. The law can only recognize reality.