 Modal Reductionism and Counterparts. Lewis parted ways dramatically  with his mentor  W. V. O. Quine  on modality. Quine (1960, §41) stands in a long line  of philosophers dating back at least to  David Hume  who are skeptical, at best, of the idea that modality is an objective feature of reality and, consequently, who question whether modal assertions in general can be objectively true or false, or even coherent. Lewis, by contrast, wholly embraces the objectivity of modality and the coherence of our modal discourse. What he denies, however, is that modality is a fundamentally irreducible feature of the world. Lewis, that is, is a modal reductionist. For Lewis, modal notions are not primitive. Rather, truth conditions for modal sentences can be given in terms of worlds and their parts; and worlds themselves, Lewis claims, are defined entirely in non-modal terms. The earliest presentation of Lewis's theory of modality (Lewis 1968) — reflecting Quine's   method of regimentation  — offers, rather than a possible world semantics, a scheme for  translating sentences in the language of modal predicate logic into sentences of ordinary first-order logic in which the modal  operators are replaced by explicit quantifiers over   worlds.[18]  The mature account of Lewis 1986 is much more semantic in orientation: it  avoids any talk of translation and offers instead a (somewhat  informal) account of concretist possible world truth conditions for a variety of modal assertions. Nonetheless, it is useful to  express the logical forms of these truth conditions explicitly in  terms of worlds, existence in a world (in the sense of AE1, of course), and the counterpart relation, which will be  discussed shortly: 