Content: wide and narrow

2. Twin earth thought experiments

A similar difficulty was revealed by Putnam’s celebrated ‘twin earth’ thought experiments (1975), which introduced the distinction between wide and narrow content (see Putnam, H.). Twin earth is a place where everything is just like it is on earth, except as otherwise specified. In one scenario, two men, Art and Bart (Art’s counterpart on twin earth), each have thoughts that they express with the words ’Water quenches thirst’. However, on twin earth the clear liquid that fills the seas and falls from the skies is composed not of H₂O but of some other stuff, XYZ. It is 1750 and Art and Bart, like everyone else, are ignorant of chemistry and could not, even if they had the opportunity, tell the difference between H₂O and XYZ. Nevertheless, Putnam contends, Art and Bart use the word ‘water’ to express different concepts. If Art were to classify a sample of XYZ as water, he would be wrong, for it would not be a substance of the same kind as water. As for Bart, he does not take XYZ to be water, for he does not have the concept ‘water’. So Art and Bart, even though they are not different neurally, have different concepts, with different conditions of correct application. Their concepts are not determined solely by what is in their heads (see Concepts §5; Methodological individualism).

Burge (1979) conducts another thought experiment designed to show that differences in people’s social environments can make for differences in mental content. An arthritic patient called Al complains, ‘My arthritis has spread to my thigh’. Nothing in his acquisition of the term ‘arthritis’ has kept him from supposing that this inflammatory disease can occur in the bones as well as the joints. Meanwhile, Cal, his twin earth counterpart, registers a similar complaint. There, however, the term ’arthritis’ is used to refer to an inflammatory disease of either the joints or the bones. Cal’s exposure to the term ‘arthritis’ is the same as Al’s, but, given how it is used on twin earth, he understands it correctly. Now, according to Burge, both patients are correctly said, on their respective planets, to believe that arthritis can occur in the bones, but, since Cal’s belief is true and Al’s is false, what they believe is different. However, there is no internal difference between them. Therefore, what they believe is partly an external matter. Contents are not, and do not supervene upon, what is in the head.

These thought experiments have met with considerable enthusiasm but also with neglected criticism (see Unger 1984; Bach 1987; Crane 1991). For one thing, they are conducted selectively: varying their details can yield contrary intuitions, for example, that XYZ is a kind of water. Also, even granting the correctness of the intuition, for example, that XYZ is not a kind of water, they leave it a mystery why the references of different concepts should be determined in different ways. The reference of the concept we express with ‘water’ depends on the nature (H₂O) of the clear, plentiful liquid around us, but this is not the situation with the concepts we express with the terms ‘earth’, ‘air’ and ‘fire’, whose references are chemically heterogeneous and are determined by the satisfaction of certain phenomenological conditions. Putnam and his followers do not explain why the fact that water is a chemical natural kind, and earth, air and fire are not, should make the concept water a different kind of concept from the concepts earth, air and fire, with its reference determined in a fundamentally different way, even back in 1750.

The arthritis argument depends essentially on the supposition that one can have beliefs with contents one ‘incompletely understands’. It assumes, for example, that Al not only misunderstands the word ‘arthritis’ but operates with the concept arthritis rather than with some broader concept (call it ‘tharthritis’) that he mistakenly associates with the word. So, it might be objected, Al understands the term ‘arthritis’ in precisely the same way as Cal does, and has the very same belief, namely that his tharthritis has spread to his thigh. Whatever evidence there is that he also believes that his arthritis has spread to his thigh is overridden by his idiosyncratic understanding of the term ‘arthritis’ (we are not tempted to say that he believes that he has inflammation of the joints in his thigh).

In any case, we cannot assume that what ‘that’-clauses capture is the sort of content relevant to psychology, that is, to characterizing people’s perspectives and explaining their actions and inferences. As Loar (1988) and Patterson (1990) have both argued, one can grant that the thought experiments succeed in showing that the truth-conditions of attitude ascriptions are sensitive to aspects of the physical and social environment without granting that ‘that’-clauses capture psychological content (see Propositional attitude statements). After all, semantics (of natural language) is not psychology.