Extract from "A Dialogue on Personal Identity and Immortality"

By John Perry. Indianapolis: Hackett Publishing, 1978.

[...] WEIROB: Let me grant for the sake of argument that belief, character, memory, and so forth are states of mind. That is, I suppose, I grant that what one thinks and feels is due to the states one’s mind is in at that time. And I shall even grant that a mind is an immaterial thing—though I harbor the gravest doubts that this is so. I do not see how it follows that similarity of such traits requires, or is evidence to the slightest degree, for identity of the mind or soul.

Let me explain my point with an analogy. If we were to walk out of this room, down past the mill and out towards Wilbur, what would we see?

MILLER: We would come to the Blue River, among other things.

WEIROB: And how would you recognize the Blue River? I mean, of course if you left from here, you would scarcely expect to hit the Platte or Niobrara. But suppose you were actually lost, and came across the Blue River in your wandering, just at that point where an old dam partly blocks the flow. Couldn’t you recognize it?

MILLER: Yes, I’m sure as soon as I saw that part of the river I would again know where I was.

WEIROB: And how would you recognize it?

MILLER: Well, the turgid brownness of the water, the sluggish flow, the filth washed up on the banks, and such.

WEIROB: In a word, the states of the water which makes up the river at the time you see it.

MILLER: Right.

WEIROB: If you saw blue clean water, with bass jumping, you would know it wasn’t the Blue River.

MILLER: Of course.

WEIROB: So you expect, each time you see the Blue, to see the water, which makes it up, in similar states—not always exactly the same, for sometimes it’s a little dirtier, but by and large similar.

MILLER: Yes, but what do you intend to make of this?

WEIROB: Each time you see the Blue, it consists of different water. The water that was in it a month ago may be in Tuttle Creek Reservoir or in the Mississippi or in the Gulf of Mexico by now. So the similarity of states of water, by which you judge the sameness of river, does not require identity of the water which is in those states at these various times.

MILLER: And?

WEIROB: And so just because you judge as to personal identity by reference to similarity of states of mind, it does not follow that the mind, or soul, is the same in each case. My point is this. For all you know, the immaterial soul which you think is lodged in my body might change from day to day, from hour to hour, from minute to minute, replaced each time by another soul psychologically similar. You cannot see it or touch it, so how would you know?

MILLER: Are you saying I don’t really know who you are?

WEIROB: Not at all. You are the one who say personal identity consists in sameness of this immaterial, unobservable, invisible, untouchable soul. I merely point out that if it did consist in that, you would have no idea who I am. Sameness of body would not necessarily mean sameness of person. Sameness of psychological characteristics would not necessarily mean sameness of person. I am saying that if you do know who I am then you are wrong that personal identity consists in sameness of immaterial soul.

[...]

[Much later]

MILLER: Let me appeal as you did to the Blue River. Suppose I take a visitor to the stretch of river by the old Mill, and then drive him toward Manhattan. After an hour-or-so drive we see another stretch of river, and I say, “That’s the same river we saw this morning.” As you pointed out yesterday, I don’t thereby imply that the very same molecules of water are seen both times. And the places are different, perhaps a hundred miles apart. And the shape and color and level of pollution might all be different. What do I see later in the day that is identical with what I saw earlier in the day?

 WEIROB: Nothing except the river itself.

 MILLER: Exactly. But now notice that what I see, strictly speaking, is not the whole river but only a part of it. I see different parts of the same river at the two different times. So really, if we restrict ourselves to what I literally see, I do not judge identity at all, but something else.

WEIROB: And what might that be?

MILLER: In saying that the river seen earlier, and the river seen later, are one and the same river, do I mean any more than that the stretch of water seen later and that stretch of water seen earlier are connected by other stretches of water?

 WEIROB: That’s about right. If the stretches of water are so connected there is but one river of which they are both parts.

 MILLER: Yes, that’s what I mean. The statement of identity, “This river is the same one we saw this morning,” is in a sense about rivers. But in a Way it is also about stretches of water or river parts.

 WEIROB: So is all of this something special about rivers?

 MILLER: Not at all. It is a recurring pattern. After all, we constantly deal with objects extended in space and time. But we are seldom aware of the objects’ wholes, but only of their parts or stretches of their histories. When a statement of identity is not just something trivial, like “This bed is this bed,” it is usually because we are really judging that different parts fit together, in some appropriate pattern, into a certain kind of whole.

 WEIROB: I’m not sure I see just what you mean yet. [...]