 Despite its seemingly outlandish nature, this twin earth thought experiment is just a way of making graphic a simple point. You and your twin are, as a matter of stipulation, molecule-for-molecule duplicates. Thus, everything situated inside your head (and biological boundaries more generally) is, by stipulation, qualitatively (if not numerically) identical. Nevertheless, your beliefs (and other propositional attitudes) differ. The principle that emerges is, then, this: if you fix what is going inside the head (and biological boundaries more generally) of an individual and vary his or her environment, some of that individual’s mental states will vary with variations in the environment even though everything in his or her head remains the same. Therefore, what occurs inside the head is, in this sense, not sufficient to determine or fix everything that occurs in the mind.