In this letter to Wellesley philosopher Mary Kate McGowan, Lewis discusses causation, explanation, and conceptual analysis in connection with dispositions.
Classification, dispositions, and conceptual analysis
Lewis tells McGowan that he thinks to have a concept of x is to have a disposition to classify possible instances of x in some way. My concept of causation allows me, for example, to separate causation from correlation. My concept of knowledge allows me to distinguish cases of knowledge from cases of mere belief. But different dispositions can interfere with each other. For instance, the disposition to be careless can scupper the disposition to multiply correctly. (There may be some influence here from Lewis’s PhD supervisor Quine. Though Lewis does not mention him, Quine often described human psychology in terms of dispositions.)
Good explanations, concepts, and classifications
McGowan appears to have asked Lewis what makes for a good (causal) explanation and about the boundaries of our ordinary concept of a cause. Lewis states that deciding what’s a good explanation is always a balancing act. Some ordinary concepts have some ambivalence in how to classify cases. Carnap had thought that this means those concepts are best replaced with cleaned-up ones. But Lewis does not agree. He thinks we ought to investigate the correct analysis of our concepts first. If we improve them at all, we should do so cautiously. In some cases, the ambivalence is informative or important to philosophy. This is true of ‘knowledge’, for example. Lewis also thinks that this is true of some vague predicates.
At the beginning of this letter, Lewis also shares some personal news with Mary Kate McGowan. His beloved cat Possum died while Lewis was in the UK and Ireland. Possum was a bit of a philosophical celebrity. Lewis often used his cat as an example in his papers.