Absences, presences and sufficient conditions

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Psychological externalism entails that certain concepts, and hence certain thoughts, are world-dependent. Gabriel Segal (2000) has argued against such a view on the following grounds.1 Let us suppose there is a place, Dry Earth, type-identical to Earth except in the following two crucial respects: first, on Dry Earth there is no watery liquid; but, second, things nevertheless appear to the inhabitants of Dry Earth exactly as they appear to the inhabitants of Earth, presumably as a result of pervasive delusions. On the assumption that the expression 'water' as used on Dry Earth expresses a concept at all, according to Segal the very same concept will be expressed by 'water' as used on Earth. If this identity claim can be established, internalism follows: supposedly world-dependent concepts turn out to be world-independent. Segal's argument for the identity claim turns on the fact that psychological properties, even according to the externalist, are supervenient properties. That is, the externalist will agree that there is a set of physical conditions on Dry Earth which suffices for possession of the concept expressed by 'water' on Dry Earth. Segal's further, and crucial, claim is that this set of physical conditions is duplicated on Earth, since everything present on Dry Earth is present on Earth. (We can grant for the moment the simplifying assumption that the only difference between the worlds is the addition of water on Earth.) It seems to follow directly that the concept expressed by 'water' on Earth is identical to the concept expressed by 'water' on Dry Earth. In my (2003) I argued that the conditions sufficient for possession of the concept expressed by 'water' on Dry Earth are not in fact duplicated on Earth. I expressed this by saying that absences themselves feature in sets of sufficient conditions. The absence of water on Dry Earth is to be counted among the conditions which suffice for possession of the concept expressed by 'water' on Dry Earth, and this absence is not duplicated on Earth; hence the conditions sufficient for possession of the concept expressed by 'water' on Dry Earth are not in fact duplicated on Earth. Harold Noonan wants to resist my argument on the grounds that it is 'the first step on a road that leads to ontological insanity' (2004: 92). Specifically, he rejects the claim that thoughts are absence-dependent, even if he accepts the notion of an absence-dependent thought as coherent. However, Noonan misconstrues, and hence fails to address, my argument. To show