 One difficulty for all these ways of invoking naturalness in the theory of rationality is that, given the popular doctrine of externalism about the content of belief, according to which duplicate thinkers in different environments can believe different propositions (see the entry on  externalism about mental content),  they lead to the much less popular doctrine of externalism about rationality, according to which duplicate thinkers can differ as regards the rationality of the inductive inferences they are engaging in. (For some arguments against externalism about rationality, see Wedgwood  2002).  For example, the discovery (by nineteenth-century mineralologist Alexis Damour) that the term ‘jade’ applies to two different minerals, jadeite and nephrite, suggests that being jade is a less natural property than, say, being turquoise. The idea that naturalness is connected to the rationality of induction threatens to imply that even before Damour’s discovery, rationality required less confidence in universal generalizations about jade (e.g. that all jade can be polished with bamboo) than it would have required for duplicate believers in worlds where all the nephrite in the actual world had been replaced by jadeite, so that the word ‘jade’ expressed the property being jadeite. Indeed, even those who embrace externalism about rationality on other grounds might not be happy with this particular instance of it: there seems to be something irrational about someone who, without the benefit of any special observations like those of Damour, is simply disposed to be more cautious in endorsing generalizations about jade than in endorsing corresponding generalizations about turquoise.