8.1 Introduction to Personal Identity

Okay, so we're going to finish today with personal identity, and as usual, we've got various luminaries there: John Locke, David Hume, Bernard Williams, and Derek Parfit. Three of them, with the exception of Hume, are closely associated with Oxford. Okay, so we start by distinguishing two different questions. First, what is it to be a person? That could invite a discussion of mind-body, the link between them, what constitutes the mind, and so forth. A different question: What is it for A and B to be the same person? That is the issue of personal identity, and it's obviously typically raised over time. What is it for a person at one time to be the same person as someone at a later time? What is it, for example, for me to be the same as I was as a baby? That we think of as the same person, is that justified, and if so, why and how? And let me start also by drawing another distinction: two different meanings of the words "same." When you say two things are the same, you can mean that they're qualitatively similar, or you can mean that they're numerically identical. And my advice would be, in all these discussions, avoid use of the word "same" just by itself because it is systematically ambiguous. So I would suggest that you use the word "similar" to mean qualitative identity when you want to say that two things have similar qualities; they look more or less the same; they have various things in common. Talk of them as similar, not as the same. And if you want to say that two things are numerically identical, say that they are "one and the same." So the claim about personal identity is that I am one and the same person as that baby, rather too many years ago now. The problem arises because of Leibniz's Law, again. We've seen this before, back in, I think, the fourth lecture. But here we are again with Leibniz's Law. It seems to be a matter of logic that if A and B are one and the same thing, then any property of A must also be a property of B. So let's call the property F. If it's true that F of A and A equals B, then it follows logically that F of B. So take these examples. Let A be me as a baby, let B be me today, and let the property F be "weighs less than a stone." We have an apparent contradiction here because I, as a baby, certainly weighed less than a stone. Today, I certainly weigh rather more. So it seems to follow that I, as a baby, cannot be numerically the same as me today, doesn't it? Well, actually, these sorts of problems are quite easily dealt with. You simply specify F more precisely. So instead of just saying "weighs less than a stone," let's put a time index in: "weighs less than a stone in 1958," which was the year I was born, as opposed to "weighs less than a stone in 2009." The fact is that I, this very person, have the property of weighing less than a stone in 1958. And you can say, if you like, that I timelessly have that property of weighing less than a stone, well, maybe a particular date in 1958. And that baby has the property of weighing more than a stone in 2009 because that very baby is me, at least if the claim of personal identity is true. So you can see that we can at least avoid overt contradiction. We don't have to say that there's some kind of serious problem in things having different properties at different times. Now, some philosophers have had difficulty seeing this. David Hume, for one. Most of Hume's treatise is actually logically very acute, but on this particular issue, he seems to have had a bit of a blind spot. He took the view that strict identity really required exact similarity. It was a major problem with saying that something that changes over time is nevertheless one and the same. In his later work, he seems to have avoided the issue, and it may be that he came to see the error of his ways. But at any rate, there, in the treatise, you will find, I think, this mistake made. Okay, so that is a mistake. We don't have to say that if something changes over time, that therefore means that it's not one and the same thing over time. Indeed, if you think about it, we want to say the reverse. I mean, a person who did not change at all would not be a person, right? A part of being a person is thinking, for example, breathing, eating, metabolizing, all the various things we do. All of those actually require change over time. It's quite impossible to breathe without changing over time. It's quite impossible to eat or talk or think without changing over time. So, far from change being incompatible with personal identity, actually, personal identity seems to require change over time. Nevertheless, there are limits. Change is defined somehow. So, we're still left with the question: What is it that constitutes personal identity over time? What is it that makes me the same person as that baby and the various intermediate stages in between? Is it physical constitution? Or is it having the same immaterial substance? Is it that when I was born (or indeed, before I was born), God implanted a soul made of immaterial substance, and somehow it's the continuity of that soul that makes me the same person? Or is it a matter of the organic life of the animal that I am, in the same way that we identify the identity of a tree as it grows over time? Maybe we are just physical organisms, and that's how our identity is constituted. Or should it perhaps be psychological continuity that makes me the same person as I was then? That there is a continuous line of psychological life? Now, notice that this question, the question I've highlighted there, "What constitutes personal identity over time?" is not the same question as asking how we judge personal identity in practice. They're likely to be closely related, but they don't have to be the same. Suppose, for example, that you did think that personal identity was constituted by an immaterial soul. Well, since immaterial souls are inaccessible to us, you might nevertheless have to say that our way of judging personal identity in practice is through the physical organism. We believe that the same soul goes along with the same body from birth until death. Therefore, the way we actually judge personal identity in practice is using the physical body, even though really, personal identity is constituted by the soul. And that way, you could quite happily say that the body is the preeminent criterion of personal identity during life, whilst continuing to believe that there you can make sense of personal identity in the hereafter through the survival of the soul.