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Mental internalists hold that an individual's mental features at a given time supervene upon what is in that individual's head at that time. While many people reject mental internalism about content and justification, mental internalism is commonly accepted regarding such other mental features as rationality, emotion-types, propositional-attitude-types, moral character, and phenomenology. I construct a counter-example to mental internalism regarding all these features. My counter-example involves two creatures: a human and an alien from ‘Pulse World’. These creatures' environments, behavioral dispositions and histories are such that it is intuitively clear that they are mentally quite different, even while they are, for a moment, exactly alike with respect to what's in their heads. I offer positive reasons for thinking that the case I describe is indeed possible. I then consider ways in which mental internalists might attempt to account for this case, but conclude that the only plausible option is to reject mental internalism and to adopt a particular externalist alternative—a history-oriented version of teleo-functionalism.