In the ‘false belief’ task (Wimmer and Perner 1983) autistics and three year olds attribute beliefs based on the objective truth of a situation in the world rather than what the person believes on the basis of available information. Several recalcitrant problems in philosophy are based on the same mistake in which philosophers take their own knowledge of the truth rather than the subject’s justified belief as relevant to belief ascription. Specifically, externalism regarding mental content rests on intuitions such as those evoked by Putnam’s Twin Earth example. However, despite its subjective force, externalism may be undermined by attending to its aetiology and showing how the intuitions arise from deceptive mechanisms. Instead of defending internalism directly, I ask: Why does externalism seem so convincing? This is a cognitive science of biases and illusions among philosophers.