Twin-earth thought experiments date back to Hilary Putnam's The Meaning of Meaning (1975). They are conducted by imagining a possible world like this in many or all phenomenal respects, but unlike it in some other way. One then compares one's own situation with that of the twin or Doppelgänger. The point of the story is to determine whether your twin or Doppelgänger shares one or another psychological state with you, it being argued that if he does not, this shows that the state is essentially ‘world involving’, i.e. its identity depends on more than the phenomenal circumstances. For example, you may think of Homer, if he existed, and we can imagine a twin on twin-earth who has the same exposure to the same stories, and who contemplates the same word ‘Homer’, but who is not thereby thinking of Homer, since Homer is not a denizen of the twin's world. See also content, wide and narrow.