The Twin Earth thought experiment was presented by philosopher Hilary Putnam in his important 1973 paper "Meaning and Reference" and subsequent 1975 paper "The Meaning of 'Meaning'", as an early argument for what has subsequently come to be known as semantic externalism. Since that time, philosophers have proposed a number of variations on this particular thought experiment, which can be collectively referred to as Twin Earth thought experiments. Edmund Husserl developed a similar thought experiment nearly 70 years earlier.
The thought experiment
Putnam's original formulation of the experiment was this:
We begin by supposing that elsewhere in the universe there is a planet exactly like earth in virtually all respects, which we refer to as ?Twin Earth?. (We should also suppose that the relevant surroundings of Twin Earth are identical to those of earth; it revolves around a star that appears to be exactly like our sun, and so on.) On Twin Earth there is a Twin equivalent of every p