The Ship of Theseus - mind, body and identity

For those who aren't familiar with this allegory, which has taken many forms, the original story was used by Plutarch to discuss identity - the ship in question was part of a legend. The ship was kept preserved by the Athenians as a monument to Theseus after he returned from Crete; as the ship inevitably decayed, its planks and other parts were replaced with new ones until, eventually, none of the original parts remained. Was it still the ship of Theseus? Heraclitus presented a similar concept with the analogy of a river - though the water that flows in it is constantly replaced, the river is still called by the same name, it has the same identity.

Let's suppose the same question could be asked of the human body: it is constantly in flux; not only are its parts constantly being replaced, but it changes structure constantly, especially the brain. Accordingly, the mind is also constantly changing. The mind which was associated with my physical brain ten seconds ago is gone, since the brain is gone, replaced by a new one. Do I still have the same identity?

I think not. There is an illusion of continuity due to the retention of memories, physical imprints on the body. But who I am now is a different person to he who existed yesterday. The consequences this has for reponsibility are not as drastic as you might think, but I can discuss this later. Does anyone disagree? Do we have a continuous intrinsic identity, or merely a 'prescribed' identity, as does the ship of Theseus?

But why does it follow that because there are qualitative differences between X and Y, that X and Y are quantitatively different? For example, according to Aristotle, if X and Y have their essential properties in common, then X and Y are one and the same object. And there are other theories of identity as well.

I'm not sure what you mean by quantitatively different.

If X and Y have their essential properties in common they have the same identity - so if two mind/body states (or people), X and Y, have their essential properties in common, then they are identical. To have the same consistent identity I need to have the same essential properties consistently.

I disagree; a thing's location in space and time is a part of its identity. Even if X and Y have the same properties, they must be different since they have a different location in space or time - if they didn't, they would be exactly the same thing. In this sense, an object's spatio-temporal locus is an essential property, one which can not be held in common by two mind/body states which are seperated in time and space, so they cannot be identical.

For those who aren't familiar with this allegory, which has taken many forms, the original story was used by Plutarch to discuss identity - the ship in question was part of a legend. The ship was kept preserved by the Athenians as a monument to Theseus after he returned from Crete; as the ship inevitably decayed, its planks and other parts were replaced with new ones until, eventually, none of the original parts remained. Was it still the ship of Theseus? Heraclitus presented a similar concept with the analogy of a river - though the water that flows in it is constantly replaced, the river is still called by the same name, it has the same identity.

Let's suppose the same question could be asked of the human body: it is constantly in flux; not only are its parts constantly being replaced, but it changes structure constantly, especially the brain. Accordingly, the mind is also constantly changing. The mind which was associated with my physical brain ten seconds ago is gone, since the brain is gone, replaced by a new one. Do I still have the same identity?

I think not. There is an illusion of continuity due to the retention of memories, physical imprints on the body. But who I am now is a different person to he who existed yesterday. The consequences this has for reponsibility are not as drastic as you might think, but I can discuss this later. Does anyone disagree? Do we have a continuous intrinsic identity, or merely a 'prescribed' identity, as does the ship of Theseus?

Let's suppose the same question could be asked of the human body: it is constantly in flux; not only are its parts constantly being replaced, but it changes structure constantly, especially the brain. Accordingly, the mind is also constantly changing. The mind which was associated with my physical brain ten seconds ago is gone, since the brain is gone, replaced by a new one. Do I still have the same identity?

There is a difference between personal identity, such as with a person or with a ship, a river, etc., and with strict identity. The latter requires that all properties are the same [(∀x)(∀y)(∀F)(Fx⇔Fy)], but the first doesn't.

I thought that personal identity was about the identity of persons. Not of ships or rivers. Why would not the identity of persons fall under Leibniz's law too? As Mickalos says?

Because I am the same person (personal continuity) as I was yesterday but it is false that I have all the properties that I had yesterday.

Not if you take Mickalos's suggestion (which is Quine's too)

I think Liebniz's law accommodates personal identity quite comfortably with the addition of a time quantifier: x and y are identical if any property possessed by x at time t is also possessed by y at time t. If you add a world quantifier it can also handle transworld identity rather well.

That works quite well for an explication of personal identity.

Many mental states are dispositional, always there just waiting to be evoked. If I ask you you your name or where you live, the answer remains the same ten seconds after the original question.

I think Liebniz's law accommodates personal identity quite comfortably with the addition of a time quantifier: x and y are identical if any property possessed by x at time t is also possessed by y at time t. If you add a world quantifier it can also handle transworld identity rather well.

That said, the meaning of, or perhaps the importance of certain aspects of personal identity change depending on context. Consider the man who is teleported to Mars, he's done it hundreds of times; he simply steps into the teleporter machine, his data is collected, his body destroyed, and then milliseconds later he steps out of a teleportation machine on Mars with all of the same memories and mental dispositions as he had on Earth milliseconds ago. Qualitatively identical. This time when he steps into the teleporter something happens and he steps out again, it seems to him just the same as all of the other times he's used the teleporter, until he realises that he's still on Earth. A scientist suddenly rushes over to him and assures him not to worry, all has gone well, there has just been a slight malfunction in the destruction of his body on Earth, but it's okay because the machine did enough to ensure that his heart will stop functioning in about five minutes. The outgoing scientist, in an attempt to reassure the man about his impending death, picks up a futuristic looking videophone, presses a few buttons, and hands hit to Earth-man, and Earth-man finds himself talking to 'himself' on mars, Mars-man, and he asks him to look after his family, and sort out all of his affairs, etc, which Mars-man, being qualitatively identical, obviously intends to do anyway.

It seems fairly obvious to me that Earth-man and Mars-man are not the same person. One is on Earth, one is on Mars, and they are even having a conversation, and in a short amount of time one of them will be dead and one of them will not be dead, and it seems to be of very little comfort to the dying man that Mars-man is psychologically-continuous with him. To Earth-man, and I think the objective observer, the conclusion should be that each time the teleporter operated one man dies and another is born, so to speak. However, when Mars-man uses the teleporter to go back to earth and goes home to his wife and children, his family obviously accept him as the same man and their lives continue as normal. A few days later at the dinner table when he mentions the strange incident of having to console his dying self, are his family going to be hit with an inconsolable anguish at the death of a father and husband? I think not, their father and husband is sitting at the dinner table with them, psychological-continuity, or qualitative identity, seems to be sufficient for them.

I don't think we mean the same, but I don't bother essentialism either.

What about this for clarification. It is a preview of my next blog post. The formatting is a bit off but the content is the same.

Identity and personal identity
Some people think that the identity notion is captured by the second proposal above. I think we need two notions of identity. I will not discuss that now.
Strict identity

For all things, for all things, for all predicates, that x and y are strictly identical logically implies that that x has predicate F is logically equivalent with that that y has predicate F.
(∀x)(∀y)(∀F)(x=Iy⇔(Fx⇔Fy)) [with obvious interpretation and =S meaning strict identity]
This is called Liebniz's law.

Personal identity
Kennethamy:[INDENT] "I think Liebniz's law accommodates personal identity quite comfortably with the addition of a time quantifier: x and y are identical if any property possessed by x at time t is also possessed by y at time t. If you add a world quantifier it can also handle transworld identity rather well."
[/INDENT]For all things, for all things, for all predicates, for all times, that x and y are personally identical logically implies that that x has predicate F at time t is logically equivalent with that that y has predicate F at time t.
(∀x)(∀y)(∀F)(∀t)(x=Py⇔(Fxt⇔Fyt)) [with obvious interpretation and =P meaning personal identity]
This seems to work.
But I don't understand the part about transworld identity. It seems to me that the above can handle transworld identity fine.

I don't think ∀x∀y∀F∀t(x=Py⇔(Fxt⇔Fyt)) is a working formalisation. For one, t could be any element in the domain, not necessarily a point in time, and F could be any two place predicate, not necessarily ending in "at time...". I can't think of a way that a traditional formulation of Leibniz's law can deal with transworld identity. If we are to make sense of modal statements about persons, like "Bertrand Russell might have been a maths teacher if he were not a philosopher", or "If Obama had lost the 2008 election, he would not now be President", then we need to be able to say that Obama in the actual world, where he did win the 2008 election, is the same person as the Obama in the possible worlds in which he didn't win the 2008 election. They don't have all the same properties (in whatever sense objects in possible worlds have properties), one is President of the United States, and the other is not. One solution might be to use something other than the idea of possible worlds to make sense of modal claims that dispenses with the problem entirely, but I don't know of any better way.

By quantitatively different I mean there are two, not one. So, if I have the "same" book as you, then it may be that you have one copy of the book, and I have another copy of the book. In that case, our books are quantitatively different, but qualitatively the same. But, if you and I have one book between us, and are sharing the book, then we have quantitatively the same book. In that case, we may say, that you and I have the identical book. That is, "one and the same book". Whether temporal/spatial location is a property of the object is controversial. Newton thought it was. But Einstein showed it wasn't. And Leibniz (who wrote a lot about identity) argued that it was not. Of course, whatever the case, some objects do not have spatio/temporal locations. The number three, which is identical with the square root of nine, has no spatio/temporal location.

I object with Kennethamy. You are stuck on the hitch that is name identity. Right now, the root thing about the ship of Theseus is originality. Does it really matter if you and i each have a copy of Harry Potter if it is the original one that is called THE harry potter? I mean, people who use proper grammar refer to their copies as "a copy of harry potter." They do not dub it "harry potter." If we refer back to the Ship of Theseus, really, as the Greeks replaced the planks and the whole of the ship, it did not become the Ship of Theseus. Since the planks were different from the original ones and basically the entire ship is different from the original ship, in truth it is NOT the Ship of Theseus. The Ship of Theseus has moved to wherever the greeks dumped the old planks and mast. When a mind and body goes, the identity should go with it. A deceased man named Bill Robinson will always be named Bill Robinson, 1921-2000, Born in the USA. Nobody should be able to take that uniqueness from hin.

I think it remained the Ship of Theseus even when the material was entirely replaced, because it was spatio-temporally continuous with the ship.