One argument sometimes heard from intelligent, widely-read, pro-life advocates runs as follows:
A fertilized ovum is, by generally agreed definition, a living cell. Moreover, it contains the full set of genes which identify it as a human cell. Being human and living, it is definitionally a human life. Deliberately and without valid excuse causing the end of a human life is murder. Hence abortion is murder.
This syllogism is seductive as it seemingly relies only on universally agreed definitions and the most basic logical steps to reach its conclusion. Yet, it also is obviously flawed for it leads to absurd conclusions. Under this definition, all individual human cells are human life. So giving someone a firm hand-shake, while knowing full-well that this is likely to cause the death of some of the recipient's skin cells, would be an act of murder.
The usual response is that this syllogism only applies to genetically unique human cells (like a fertilized ovum), not all of them. That is not entirely satisfactory as genetic uniqueness nowhere enters that syllogism. But even if it was accepted, it would not remove all absurd consequences.
Consider a murderer of only one of a pair of identical twins. Or a murderer who with some medical skill and equipment keeps, for example, the kidney of the victim alive. Nobody would argue that this means that no murder took place, yet it follows from the amended syllogism.
Or imagine yourself giving blood. As the bag of blood is nearly full, a man breaks into the room, pulls out a gun, takes aim, and fires one shot. The man's behavior is certainly disagreeable and illegal. Most likely it is even criminal. But it makes a rather large difference whether the man shot you or the bag of blood. In either case, he would have deliberately caused the death of all human cells in one set (you) or another (the bag of blood). In neither case, would he have caused the death of all (or the last) cell of your unique genetic combination for some of the cells with your genetic code survive, either the bag of blood or you. Yet in only one case—shooting you—has the man committed murder.
The solution is that murder is not deliberately causing the death of anything that is human and alive (whether genetically unique or not). Rather, murder is deliberately causing the permanent extinction of human thought.
This definition is free of these absurdities. The man who shoots you is a murderer. The man who shoots the bag of blood is merely a lesser miscreant. The man who kills one of a pair of identical twins is a murderer. So is the man who keeps his victim's kidney alive.
Some of the other consequences of this definition may be controversial, but also strike me as correct. A doctor who medically induces a coma, temporarily extinguishing human thought, is not a murderer. Even should the procedure go awry and the patient die, resulting in a permanent extinction of human thought, it would still not be murder as the permanent extinction was not deliberate. A doctor who induces the death of a body in an irreversible coma is also not a murderer because the permanent extinction took place before the doctor acted and hence was not caused by him; depending on other circumstances, he may be guilty of lesser offenses.
Applying this definition to the abortion question yields a mixed verdict. Very early abortions before a brain has started to develop are clearly not murder. Conversely and equally clearly, human thought of a primitive kind does not spring suddenly into existence at birth, but develops some time before. So late-stage abortions are murder and should be treated as such. That does not necessarily mean that there would be criminal punishment; certain defenses1 would apply in some rare circumstances, such as when the mother's life is at genuine risk, much like they do in any other murder case.
Abortions of a disabled foetus would be murder it the foetus has developed the ability of thought to some basic level. But abortion of a foetus with a disability so profound that it cannot develop thought, like anencephaly, would not be murder.
This definition of course does not lead to an exact date at which abortion becomes murder. One can reasonable disagree about what level of cognition constitutes human thought. The capacity for cognition develops gradually, rather than in discrete jumps. As our knowledge of fetal development advances, one's conclusion about the point in time at which human thought exists may vary.
In general, though, it can be said that under this definition abortion would be legal in first trimester, illegal in the third, and change its status sometime in the second. That is a conclusion agreeable to this author and, in a rare coincidence, apparently also public opinion as expressed in surveys.
1 The applicable doctrines would not be self-defense or defense-of-others. For these doctrines to apply, the accused must prove that the dead person intended to commit a very serious crime against the accused (or another) and was about to put that intent into effect, or at least that the accused reasonably so believed. But surely no unborn child—or even a young, born child—could form any such criminal intent. Other doctrines, such as the defense of necessity, have no such requirements and hence may apply.
