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Suppose Davidson goes hiking in the swamp and is struck and killed by a lightning bolt. At the same time, nearby in the swamp another lightning bolt spontaneously rearranges a bunch of molecules such that, entirely by coincidence, they take on exactly the same form that Davidson's body had at the moment of his untimely death. This being, whom Davidson terms "Swampman," has, of course, a brain which is structurally identical to that which Davidson had, and will thus, presumably, behave exactly as Davidson would have. He will walk out of the swamp, return to Davidson's office at Berkeley, and write the same essays he would have written; he will interact like an amicable person with all of Davidson's friends and family, and so forth.
Davidson holds that there would nevertheless be a difference, though no one would notice it. Swampman will appear to recognize Davidson's friends, but it is impossible for him to actually recognize them, as he has never seen them before. As Davidson puts it, "It can't recognize anything, because it never cognized anything in the first place."
These considerations lead Davidson to deny that the Swampman's utterances can be construed as referring to anything in particular. To take a fairly specific example, suppose that at some point the previous day Davidson had looked at a glass marble on a shelf; unbeknownst to him there was another, visually identical, glass marble hidden right behind it. When he makes an assertion about "the marble I saw yesterday," we take him to be referring to the one that he did in fact see, even if he could not supply enough descriptive information to identify it later. Had the marbles been arranged in the other order, therefore, we would take him to be referring to the other marble, yet his internal state would be identical.
The Swampman has no causal history. He is in the same state as the actual Davidson and the counter-factual Davidson considered above, whose utterances refer to different marbles. As a result his utterance could refer to either marble. In principle, Davidson tells us, the above indeterminacies can be extended to any degree we like: the fact that the Swampman happens to be identical to Davidson does not change the fact that he could have arrived at that state by any one of countless histories, each of which would demand we interpret him differently. Until the Swampman has begun interacting with and using language among the objects of the real world, we can have no grounds to attributing any meanings or thoughts to him at all.
Amongst those who accept the force of this argument, there are two distinct ways of viewing its consequences. On the one hand, many philosophers have taken it to affect merely how we should evaluate Swampman. The argument is believed to demonstrate that Swampman's utterances and thoughts do not mean anything, and do not refer to anything in particular. On this view, Swampman's subjectivity and consciousness are considered to be unchanged. Others have argued that this lack of a causal history renders incoherent the notion that Swampman could have a mind at all, which in turn raises the question of whether he is, in fact, a person. (Note that Davidson calls Swampman "it" rather than "he.")
This argument depends upon the acceptance of semantic externalism - the claim that what one's words mean is determined not merely by some internal state, but also by the causal history of the speaker.
- Other philosophers[who?] have denied that any such conclusions can be derived from this thought experiment. Many adherents to the mind-brain identity theory have maintained that, as Swampman's brain is identical to Davidson's, so must his mind be identical to Davidson's. Therefore, any theory of language which leads one to conclude that Swampman's utterances and, particularly, thoughts, are any different from Davidson's is necessarily flawed.
- Daniel Dennett, in Consciousness Explained, has called into question the validity of this sort of thought experiment altogether, maintaining that when a thought experiment is too far removed from the actual state of affairs, our intuitions cease to be meaningful.
- A third point of objection is that Davidson and the Swampman become individuals in their own rights immediately when they make choices of their own. Regardless of their physical make-up, neither can be defined in any way as individuals until their choices are considered. Thus, the two are only identical people insofar as they make their own choices-—something which the thought experiment does not consider.
This concept has been said to have implications for mind uploading or mind transference, as it indirectly addresses the question of whether an exact replica of one's brain patterns, created within an alternate brain, an android brain, or a virtual universe, is the same being as the person it was copied from, or another being entirely.
See also 
- Twin Earth thought experiment
- Philosophical zombie
- Gareth Evans
- Identity and change
- Philip K. Dick
- Ship of Theseus
- The Terminal Experiment
- Omphalos hypothesis
- Mind uploading
- Davidson, Donald (2001 (1987)). "Knowing One's Own Mind." Reprinted in Subjective, Intersubjective, Objective (pp. 15–38). New York and Clarendon: Oxford University Press. Originally published in Proceedings and Addresses of the American Philosophical Association, 60 (1987), 441-58.