Externalism about mental content is now widely accepted. It is therefore surprising that there is no established definition of externalism. I believe that this is a symptom of an unrecognized fact: that the labels 'mental content externalism' -- and its complement 'mental content internalism' -- are profoundly ambiguous. Under each of these labels falls a hodgepodge of sometimes conflicting claims about the organism's contribution to thought contents, the nature of the self, relations between the individual and her community, and the (...) epistemic availability of thoughts. This situation stems from the fact that contributors to debates about externalism differ in how they understand 'internal property'; these differences reveal (or, perhaps, generate) disparate conceptions of what is at issue in these debates. I argue that this situation is irremediable. There is no way to understand 'internal property' that will conform with prevailing beliefs about the nature of internalism and externalism, and with the usual taxonomy of leading positions. This ambiguity carries a heavy price: participants in these debates often argue at cross-purposes, disagreeing even on the nature of the evidence that could settle the question of externalism. Progress on the broad range of issues associated with these debates requires that we abandon the categories 'internalism' and 'externalism'. I close by suggesting a promising avenue for future research related to these issues. (shrink)