Bodily Identity

Bodily Identity

Suppose you are interested in the nature of bodily survival: under what conditions does a body survive into the future? Your initial theory is that identity through time is a necessary and sufficient condition: the body then must be numerically identical to the body now. This is generally how it is and you conjecture this is how it must be: for example, a cat’s body will only survive if some future body is identical to that body; if no future body is identical to the given body, it will not count as that body surviving into the future. But now you notice a curious fact: some bodies can be bisected and the parts go on living. Some organisms divide, or can be divided, in two and the results survive into the future. Is there an identity between the original body and the surviving results of bisection? No, because neither of the two bodies is identical to the original body, since they are not identical to each other; and they can’t be identical to the original as a composite because the composite consists of different functional bodies and the original was not such a pair (the two bodies have grown extra limbs where none existed before). Yet we would want to say that the original survives the operation—bisection isn’t the same as death by obliteration. That is, in fission cases we have survival without identity. Fission is a better fate than death, if you care about bodily survival. It seems, then, that your initial theory of bodily survival was wrong: survival of the body does not logically require identity through time.

It has been claimed that survival of the person follows the same logic: a person can survive an operation in which no future person is identical to the original. These are brain bisection cases: you cut the brain into two and place the halves in two bodies, thus producing two persons not identical with any past person. Personal survival does not require personal identity, as these fission cases show. The human brain is made of two hemispheres each capable of housing a person if bisection is performed. The person can be divided from himself, as it were, as the body can be divided from itself. The anatomy of the brain makes that possible, as the anatomy of our hypothetical organism makes a similar thing possible. What metaphysical conclusions can we draw from such fission cases? What we cannot conclude is that persons are distinctively capable of survival without identity. We cannot say that it is a defining property of persons that they are able to survive without identity holding between them and some future person. It isn’t as if persons differ from bodies in this respect. It just turns out that in viable bisection cases we get survival without identity, whether it is persons or bodies that are at issue. We have made no such discovery about persons and only persons. So, we have not laid the groundwork for the further conclusion that personal survival consists of psychological continuity and connectedness as opposed to simple identity—unlike other classes of entity. In fact, the cases are precisely parallel: there is nothing particularly tenuous and degree-like about the survival of persons over time, as a matter of metaphysical necessity. It isn’t that personal survival is uniquely non-identity-requiring. In fact, this is a quite banal truth about survival in general—a matter of simple logic and the possibility of dividing things into two. It isn’t that persons possess the remarkable property, alone in the universe, of being capable of surviving in the absence of identity.

Let’s add another thought experiment: consider a possible world in which brains are not divisible in the way they are in the actual world—if you cut them in two, both halves die on the instant. Then there will be no possible fission cases in that world: there cannot be survival without identity of persons in that world. And yet there may still be fission cases for bodies in this world. Then we will have survival without identity for bodies but no survival without identity for persons. We invert the usual thought experiment. Teachers of philosophy in this world will be able to provide convincing thought experiments of the former type (bodies) but not the latter type (persons). Students will be persuaded that bodies can survive in the absence of identity, but not that persons can. If the latter is suggested, they will retort that such a thing is impossible, because brains cannot be divided without causing death. Bodies can, though, as illustrated by the organisms that do survive under fission; but persons can’t, because brains can’t survive under fission. It will be possible to write books about bodies drawing attention to their loose survival conditions, but not write such books about persons—they can survive only under conditions of strict identity.

Is it, then, that the correct theory of personal survival depends on the kind of world you are living in? If you are living in a world in which brains are like current human brains, then the non-identity continuity-and-connectedness theory holds; but if you are living in a world in which brains are not like this, then the simple identity theory holds. That would mean that there is no metaphysical truth about persons as such—only a kind of relative truth. It is only human persons (or those anatomically like them) that allow survival without identity, and hence invite a continuity-and-connectedness theory of personal survival; other species might not permit of the kind of operation envisaged by defenders of that theory. Indeed, it might be that the universe contains many species of persons all but one of which cannot survive without identity. This is not what proponents of the non-identity theory of survival had in mind—they purported to speak of the nature of persons not of some specific biological type of person. You can’t make metaphysics out of biological contingency.[1]

[1] I am alluding, of course, to the influential views of Derek Parfit.