David Lewis is notoriously known for believing in the existence of concrete possible worlds equally real as our own. It came to be known as (genuine) modal realism. His systematic presentation and defence of the view appeared in On the Plurality of Worlds (Blackwell, 1986). But when exactly did he adopt the view? And what exactly was the original motivation? In this letter to W.V. Quine we get the answer!? Lewis’ letter is in response to Quine sending his Foreword for Lewis’ first book Convention (Harvard UP, 1969). Lewis is reacting to Quine’s final paragraph on p. xii. There Quine writes:

The problem of distinguishing between analytic and synthetic truths was apparently one motive of the study. In the end, Lewis concludes that the notion of convention is not the crux of this distinction. He does not for this reason find the analyticity notion unacceptable, however. On the contrary, he wonders why I questioned the notion in the first place, any more than the notion of truth. This is not the place to repeat my reason for regarding truth as much the firmer notion; anyway, he must have found my reason unconvincing. He ends up rather where some began, resting the notion of analyticity on the notion of possible worlds. His contentment with this disposition of the analyticity problem makes one wonder, after all, how it could have been much of a motive for his study of convention; but we may be thankful for whatever motives he had. For in the course of the book the reader comes to appreciate convention, not analyticity, as a key concept in the philosophy of language (1969, p. xii).

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