Suppose Don goes hiking in the swamp and is struck and killed by a lightning bolt. At the same time, elsewhere 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 Don's body had at the moment of his untimely death.
This "swamp-man" will walk out of the swamp, return to Don's office at Berkeley and write the same essays he would have written; will interact like an amicable person with all of Don's friends and family, and so forth. Does it in fact have a mind? When it makes English-like noises, is it saying things? Is it a person?
At first blush we would be inclined to affirm all these things. But take a fairly specific example. Suppose at some point the previous day Don had looked at a glass marble on a shelf; unbeknownst to him there was another, visually identical glass marble hidden right behind it. When Don says, "that marble I saw yesterday," he is referring to the one that he did see, even if he could not supply enough descriptive information to identify it later. This is because in judging the reference of a word we take into account the causal history of its user. It is possible that the marbles might have been in the opposite positions when Don saw them; if they had been, he would have been referring to the other one. Furthermore it is possible that if this had happened Don's physical state might have been utterly identical to the way it in fact turned out. This means that the physical state of a person is not sufficient to fix the meanings of his words. We must take his causal history into account.
The swamp man has no causal history of the sort that would allow us to determine what his words mean. In principle, the philosopher Donald Davidson tells us, the above indeterminacies can be extended to any degree we like: the fact that the swamp man 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 Swamp-Don has begun interacting with and using language among the objects of the real world, we can have no grounds to attributing any mental state at all to him.
This is essentially an argument for externalism as regards meaning - the claim that what one's words mean is determined not by some internal state, but rather by the causal history of how the words were acquired by the speaker.
See also: reference, Gareth Evans