Sufficient Absences

Externalism about psychological kinds is the thesis that certain of a subject's mental states and events are dependent for their individuation on the subject's environment. The thesis opens up the possibility that a subject's mental state and event kinds might vary with variations in the subject's environment, even while her physical properties, including her functional properties and her physical history, all individualistically and non-intentionally described, remain constant. Hence psychological externalism opens up the possibility that physically indistinguishable subjects could nevertheless be distinguishable psychologically. In this paper I will examine one argument against natural kind externalism. However, my interest is not primarily either with the particular argument, or even with the defence of externalism against it. Rather, I am concerned to show that reflection on the general form of the argument brings to light a surprising and important fact about supervenience, namely that subvenient bases must be construed as involving absences. This fact serves, of course, to undermine the particular argument against natural kind externalism I deal with. Let me begin with the following pressing question: What is the externalist to say about apparent natural kind terms that fail to refer, such as 'phlogiston' and 'ether'? The problem becomes apparent when we consider physical duplicates one of whom has a concept that refers to a natural kind, 2 and the other of whom lives in a world where there is no plausible such natural kind to which any putative twin concept might refer. Consider the following example taken from Paul Boghossian (1997). We are to suppose that there is a planet, Dry Earth, similar to Earth in all respects in which Twin Earth and Earth are similar to one another, but which differs from both in that, although it very much seems to its inhabitants that there is a clear, tasteless and colourless liquid flowing in their rivers and taps and to which they confidently take themselves to be applying the word 'water', these appearances are systematically false and constitute a sort of pervasive collective mirage. In point of actual fact, the lakes, rivers and taps on this particular Twin Earth run bone dry. (1997: 170) The question now becomes: does the word 'water' as used on Dry Earth express a concept, and if so, what concept does it express? Gabriel Segal uses this question to argue against externalism. He writes, The externalist now faces a dilemma. He must either deny …