Version: v1, Published online: 1998
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2. Physical criteria
Physical criteria: the bodily criterion. According to the bodily criterion, person A at time t1 is identical to person B at t2 if and only if A and B have the same body (that is, they are bodily continuous). Note that A and B can truly be said to have the same body, even though the body at the later time has no matter in common with the body at the earlier time (see Continuants). In such a case, however, the replacement of matter must be gradual, and the new matter must be functionally absorbed into the living body. This is how it is in the life of a normal human being.
The bodily criterion accords with most of our ordinary judgments of personal identity. However, there appear to be logically possible cases in which the deliverances of the bodily criterion conflict with our considered judgments. The particular case I have in mind is that of brain transplantation. Such transplants are, of course, technologically impossible at present; but that is hardly relevant. The speculations of philosophers are not confined to what is technologically possible.
Sydney Shoemaker was the first to introduce such cases into the philosophical literature. He wrote:
It is now possible to transplant certain organs… it is at least conceivable…that a human body could continue to function normally if its brain were replaced by one taken from another human body.…Two men, a Mr. Brown and a Mr. Robinson, had been operated on for brain tumors, and brain extractions had been performed on both of them. At the end of the operations, however, the assistant inadvertently put Brown’s brain in Robinson’s head, and Robinson’s brain in Brown’s head. One of these men immediately dies, but the other, the one with Robinson’s head and Brown’s brain, eventually regains consciousness. Let us call the latter ‘Brownson’.…When asked his name he automatically replies ‘Brown’. He recognizes Brown’s wife and family…, and is able to describe in detail events in Brown’s life…of Robinson’s life he evidences no knowledge at all.
(Shoemaker 1963: 23–4)
We can suppose, in addition, that Brown and Robinson are physically very similar, and that their bodies are equally suited for the realization of particular dispositions or abilities (for example, playing the piano, or hang-gliding).
The description of this case which commands almost universal assent is that Brown is the same person as Brownson. Virtually no one thinks that the correct description is: Robinson acquires a new brain. Receiving a new skull and a new body seems to be just a limiting case of receiving a new heart, new lungs, new legs, and so on. If Brown is the same person as Brownson, and yet Brownson’s body is not the same body as Brown’s body, then it follows that the bodily criterion is false.
Physical criteria: the brain criterion. In the light of this example, it would be natural for a defender of the physical criterion to move to the brain criterion: A at t1 is the same person as B at t2 if and only if A and B possess the same brain. But is this a plausible criterion of personal identity? I think not. The following scenario is conceivable. Imagine that robotics and brain science have advanced to such a stage that it is possible to construct a silicon brain which supports the very same kind of mental life as that supported by a flesh-and-blood human brain. Imagine also that parts of a human brain (say, a cancerous part) can be replaced by silicon chips which subserve the very same mental functions as the damaged brain tissue.
Suppose that the whole of my brain gradually becomes cancerous. As soon as the surgeons detect a cancerous part, they replace it with silicon chips. My mental life continues as before – the same beliefs, memories, character, and so on, are preserved. Eventually, the surgeons replace all my biological brain with a silicon brain. Since my mental life, and physical appearance and abilities, are unaffected by this replacement, we have no hesitation in judging that I have survived the operation. The procedure preserves personal identity. But is this judgment of personal identity consistent with the brain criterion? The answer to this question depends on whether my (later) silicon brain is deemed to be identical to my (earlier) human brain.
It is plausible to suppose that, if an object (such as a heart, brain or liver) is biological, then that very object is essentially biological. That is, for example, my flesh-and-blood brain could not have been anything but a biological entity. This essentialist thesis is consistent with the view that the function of any given biological object (a human heart, for example) could, in principle, be carried out by a non-biological object (a mechanical pump, say). Hence, I am happy to concede that my later silicon brain is indeed a brain; but it is not remotely plausible to think that it is the same brain as my earlier human brain. Rather, the effect of all the tissue removals and bionic insertions in my skull is to destroy one brain and replace it with another.
Our brain example shows that the sort of matter or stuff with which we replace an object’s removed parts can affect the overall identity of that object, even if continuity of form or function is preserved. My (earlier) human brain is not identical to my (later) silicon brain. Yet I survived the operation. Hence, the brain criterion is false.
However, there is a deeper worry about the tenability of the brain criterion. Why did we move to the brain criterion, in response to counterexamples to the bodily criterion? Was it because the human brain is a three-pound pinkish-grey spongy organ that occupies human skulls? No, we moved to the brain criterion because of what the human brain does, namely, supports directly our mental life. It is surely because of its mind-supporting function that we are inclined to single out the brain as the seat of personal identity. Consequently, we should not see our identity over time as tied necessarily to the continued existence of the human brain we presently have. What matters most is that our stream of mental life continues to be supported by some physical object, not that it continues to be supported by the very same biological organ.
Garrett, Brian. Physical criteria. Personal identity, 1998, doi:10.4324/9780415249126-V024-1. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/personal-identity/v-1/sections/physical-criteria.
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