Identity Over Time and The Ethics of Abortion


There is a well-known philosophical argument against abortion that focuses on the identity of the embryo and fetus over time. Alex Pruss presents a prototypical example of this kind of argument in his paper I Was Once a Fetus: That is Why Abortion is Wrong.

The argument can be summarized as follows: Every born human being was once an embryo. Our personhood is an essential property of who we are and hence, we are persons at every moment of our existence. Therefore, we were persons at our embryonic stage and so abortion kills innocent persons making it prima facie wrong.

The crucial premise in this argument is the identity thesis: human persons are essentially human organisms. This thesis must be true for embryos to be one and the same entity as future people. For the purposes of this post, all I mean by “person” is “an individual of our essential kind,” whatever that kind may be (human animal, embodied mind, Cartesian ego, Lockean person, etc). With that in mind, I will argue that the identity thesis is false.

But first, notice the nature of the identity thesis. The thesis is not purely biological in character. It is distinctly metaphysical in character. There is no dispute about scientific questions such as whether embryos are alive, whether embryos are human, or whether embryos have the same DNA as future persons. All of that is agreed upon. The question is whether the metaphysical identity of the embryo persists such that all born persons are identical with past embryos. That is a question that can only be answered through philosophical argument. Scientific knowledge, by itself, is not sufficient.

This might sound odd to some, so perhaps an analogy would help. Just as there is no scientific experiment that could tell us whether a statue and the lump of bronze of which it is composed are one and the same thing or distinct substances, there is no scientific experiment that can determine whether a embryo is the same thing as a future person. Both debates fall squarely in the realm of metaphysics.

With this backdrop, I will present 3 arguments against the identity thesis

Brain Death

Human persons are not essentially human organisms because human persons can cease to exist while their bodies remain alive. Since the human organism exists and the human person no longer exists, then human person cannot be identical to the human organism.

Most of the time, brain death occurs with the death of the whole body. However, this isn’t always true. In some rare cases, the brain dies while the body remains alive.

In such a case, intuitively, the person is gone, but the human organism is still alive. If you permanently obliterate all of my beliefs, desires, memories, goals, feelings, and emotions, then I no longer exist. I am no longer “in” my body even if my body is biologically alive.

Think about it like this: If someone were to strike your mother in such a way that she experiences brain death, you would want that person charged with murder, not merely assault. You would want murder charges even if his body remained biologically alive.

To drive this point home, consider a serial killer who gives you two options: (A). He will cause you to experience brain death, but your body remains alive (B). He will shoot you in the head and kill you directly on the spot.

We are also stipulating that in both cases, your body will be hidden away from your family, friends, etc. In other words, there are no relevant extrinsic factors that can favor one option over the other. Intuitively, there doesn’t seem to be much reason to pick option A over option B There is no difference between the two cases from a first person perspective(since there is no first person perspective in either case)

Indeed, let’s say you go with option A. Has the serial killer acted any better, from a moral perspective, than he would have if option B had been chosen instead? That is, is causing brain death(but keeping the body alive) any better, morally, than just killing someone directly on the spot? Certainly not. That is, both seem like murder in the morally relevant sense of the word. The person has been “killed” in both cases.

Hence, we have strong reason to believe that human persons do not “survive” brain death. But if that’s true, then the identity thesis is false.

Monozygotic Twinning

If the identity thesis is true, then a morally valuable person ceases to exist in the process of monozygotic twinning. When an embryo divides to form twins, if the divisions symmetrical, the original embryo also ceases to exist. This point is made by philosopher Jeff McMahan in the paper Killing Embryos for Stem Cell Research.

The original embryo cannot be identical with both twins, since one thing cannot be numerically identical with two things that are not identical with each other. And if the division is symmetrical, the original embryo cannot be one twin but not the other, for there is nothing about one twin to identify it as the original embryo that is not also true of the other.

If the embryo is identical to a human person like you or me and if it matters in the way you and I do, then when monozygotic twinning occurs and an embryo ceases to exist, this should be tragic since it is the ceasing to exist of someone who matters. But this is absurd. No one believes that having twins is some sort of great moral misfortune, but that is precisely what the identity thesis entails.

Conjoined Twinning

As Jeff McMahan has also noted, dicephalus conjoined twinning appears to be a counterexample to the identity thesis. Dicephalus occurs when a human zygote divides incompletely, resulting in twins fused below the neck. In some cases, there is one circulatory system, one metabolic system, one reproductive system, and one immune system. Said another way, there are two persons but only one human organism.

Dicephalus is a counterexample because each twin is a person and is related to the organism in the same way as the other; therefore, there is no basis for the claim that one is the organism and the other not (though even the claim that one of them is the organism would imply that at least one person is not an organism)

Given the transitivity of the identity relation, both persons cannot be identical to the organism without being identical to each other. Since they are not identical to each other, it follows that neither is identical to the organism.

Therefore, there are persons that are not essentially organisms, so the identity thesis is false. Moreover, since each dicephalic twin is the same kind of entity that we essentially are, none of us is essentially an organism.

If Human Persons Aren’t Human Organisms, Then What Are They?

If the identity thesis is false, then one naturally wonder what human persons essentially are. I think the answer is provided in Jeff McMahan’s embodied mind account of personal identity(The Ethics of Killing, 66)

According to the account, the central criterion of personal identity is the continued existence and functioning, in non-branching form, of enough of the same brain to be capable of generating consciousness or mental activity. The embodied mind account avoids all of the previously mentioned counterintuitive implications of the original identity thesis.

We can further support the embodied mind account by appealing to a thought experiment offered by McMahan. McMahan explains, “Suppose that you and your identical twin are both involved in a terrible accident. Your brain is undamaged, but the rest of your body is so badly injured as to be moribund. Your identical twin’s brain has been destroyed, but the rest of his or her body is undamaged. Exploiting new techniques that enable the proper neural connections to be made between your brain and your twin’s body, your surgeons remove your twin’s dead brain and transplant your perfectly functional brain in its place”(Killing Embryos for Stem Cell Research, 181-182)

Almost everyone would agree that the person who then wakes up in that body is you. But if you were a human organism, you would now be the dead organism from which your brain was extracted, and the person who wakes up after the surgery would be your twin, an erroneous conclusion.

This thought experiment also supports the notion that retaining the enough of same brain over time is necessary for personal identity to be preserved, giving even more reason to accept the embodied mind account.


There is more that sufficient reason to reject the identity thesis and the anti-abortion argument that derives from it. There is an account of personal identity, the embodied mind account, that provides a far superior explanation of our intuitions in both real and hypothetical cases. Importantly, the embodied mind account does not entail an anti-abortion conclusion.

Given this, considerations about personal identity are of no help to pro-life advocates. The moral permissibility of abortion remains without any serious challenge.

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