Swampman is the subject of a philosophical thought experiment introduced by Donald Davidson in his 1987 paper "Knowing One's Own Mind".

The experiment runs as follows:

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 counterfactual 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 the meaning of one's words is determined not merely by some internal state, but also by the causal history of the speaker.

Davidson holds that there would "nevertheless be a difference", though no one would notice it. However he did not provide further elaboration and justification for the assertion. Objectors hold that there would be no difference.

One 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, which is something that the thought experiment does not consider.

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.

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 alternative brain, an android brain, or a virtual universe, is the same being as the person it was copied from, or another being entirely. The same question is to be considered if a technology to teleport people arises.