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This month’s letter is from September 1989, from Lewis to Richard Cartwright. In the Introduction to his Philosophical Essays (MIT Press, 1987), Cartwright takes issue with Lewis’s claim (in his own Philosophical Papers, vol. I, OUP 1983, xi) that ‘once the well worked-out menu of philosophical theories is before us, philosophy is a matter of opinion’. According to Lewis, there are — perhaps only occasionally — irresolvable disputes about philosophical claims. In such cases, there is no agreed set of evidence or methodological norms that will enable one side to convince the other on terms that are acceptable to both. Lewis concludes that the best we can aim for, individually, is to reach ‘equilibrium’ with respect to our own opinions. Cartwright is ‘left uncomfortable’. At least part of this discomfort seems to stem from the assumption that Lewis’s ‘opinions’ are mere opinions: claims that cannot be known or for which we lack justification — and it is this charge in particular that Lewis takes issue with in his letter.

Cartwright ends his discussion by saying: ‘I have been protesting the idea that philosophical differences are ever in the end differences of opinion, and the associated idea that to philosophize on any subject is in the end to choose from an array of theories no one of which has credentials superior to those of any other. I do not, of course, mean to disparage the quest for equilibrium. But I do not envy those who think they have reached it by coming to rest in some philosophical system. Apart from occasional tinkering, they have nothing left to do except spread the word. And why should they want to do even that? The thrill must be gone’ (1987, xv). My guess is that Lewis’s response that one such theory can, in fact, have superior credentials to another but only by one’s own lights will not have made Cartwright feel much less uncomfortable — though so far as I can tell, Cartwright didn’t offer any alternative solution to the problem of ‘ultimate impasses’.