Some examples devised by philosophers are weird but analytically very useful. Take this one by Donald Davidson:
“Suppose lightning strikes a dead tree in a swamp; I am standing nearby. My body is reduced to its elements, while entirely by coincidence (and out of different molecules) the tree is turned into my physical replica. My replica, Swampman, moves exactly as I did; according to its nature it departs the swamp, encounters and seems to recognize my friends, and appears to return their greetings in English. It moves into my house and seems to write articles on radical interpretation. No one can tell the difference.
But there is a difference. My replica cannot recognize my friends; it cannot recognize anything, since it never cognized anything in the first place. It can’t know my friends’ names (though of course it seems to); it can’t remember my house. It can’t mean what I do by the word ‘house’, for example, since ... [it] was not learned [by Swampman] in a context that would give it the right meaning – or any meaning at all. Indeed, I don’t see how my replica can be said to mean anything by the sounds it makes, nor to have any thoughts.” (source: see below; italics in the original)
Much can be said about this example and much has been said about it. I think that it gives some answers but raises many questions, too. Anyway, I think that you’ll agree that Swampman is not Davidson.
Take now this case by Parfit: “I enter the Teletransporter. ... This machine will send me at the speed of light [to Mars]. I merely have to press the green button. ... When I [do], I shall lose consciousness, and then wake up [an hour] later. ... The Scanner here on Earth will destroy my brain and body, while recording the exact states of all my cells. It will then transmit this information by radio ... [to] the Replicator on Mars. This will then create, out of new matter, a brain and body exactly like mine. It will be in this body that I shall wake up.” Parfit presses the green button and wakes up on Mars: “Examining my new body, I find no change at all.” (source: see below)
Next Parfit discusses the relevance of his example for the problem of personal identity. However, is it possible to be teletransported in this way? Parfit’s example suggests that the answer is yes, but after having read Davidson, the answer is clear: No. From Davidson’s discussion we can learn that Parfit on Mars is not a kind of resumption of Parfit on Earth. For instance, what Parfit’s Mars-Replica knows was not learned by him in a context that would give it the right meaning (see above). It’s simply a copy, just as a copy of a letter is a copy of a letter and not the original, even though it has the same contents and the same layout. Moreover, in the case of the teletransport the original has been destroyed, just as Davidson has been by the lightning in the Swampman example. There isn’t even a (psychological) continuity between Parfit on Earth and his replica on Mars, as Parfit thinks, for there is no logical necessity that Parfit on Earth must be destroyed; just as we don’t need to destroy the original letter once we have copied it. A copy is not a continuation of the original but duplicate of it.
However, copying Davidson to Swampman or Parfit to Replica-Parfit doesn’t need to happen all of a sudden. Think of the Ship of Theseus: Theseus returns from Crete to Athens, after having killed the Minotaur, and has to repair his ship at sea. He replaces the old planks of the ship one by one by new ones so that finally none of the old planks of the ship that left Crete remains. Then the question is: Is the ship that arrives in Athens the same one as the ship that left Crete? To my knowledge there has never been given a satisfactory answer to this question. If one looks at the ship when it left Crete and then again when it arrived in Athens only, one tends to say “no” in view of Davidson’s case. On the other hand, I think that the sailors had always the idea that they used the same ship. And how about halfway Crete and Athens? The questions become even more intriguing, if you realize that man is like the Ship of Theseus: Man is continuously under construction and reconstruction. Man is continuously repaired and renewed and after some years none of the molecules we originally consisted of are yet the same. Then the question is: Are we the same as we are? You know the answer for yourself but it seems that the Swampman example is not as weird as it might seem on the face of it. Think about it, and the more deeply you go into it, the more you’ll discover in it.
- Davidson, Donald. “Knowing One's Own Mind”, in his Subjective, Intersubjective, Objective. New York: Oxford University Press, 2001; p.19.- Parfit, Derek, Reasons and Persons. Oxford: Clarendon Press, 1984; pp. 199, 215.