 To illustrate the dispute, consider two physically identical cognitive systems A and B. Among the symbols processed by A is symbol S. A produces instances of S whenever A is in front of bodies of water, when A is thinking of water, when A is forming plans to interact with water, and so on. In short, symbol S appears to stand for water. Every time A processes S, system B processes symbol S′, which is physically identical to S. But system B lives in an environment different from A’s environment. Whenever A is surrounded by water, B is surrounded by twin-water, which is a substance superficially indistinguishable, yet physically different, from water. Thus, symbol S′ appears to stand for twin-water (cf. Putnam 1975b). In summary, A and B live in relevantly different environments, such that S appears to stand for water while S′ appears to stand for twin-water; A processes S in the same way that B processes S′; and there is no intrinsic physical difference between A and B or between S and S′.