David Lewis [11] 84 has claimed that the provision of an adequate explanatory semantics for statements involving the modal notions of (logical) necessity and possibility requires the postulation of possible worlds. It has been suggested, however, that without abandoning any advantages of a possible worlds semantics, we can avoid Lewis' highly counter-intuitive metaphysic by reducing possible worlds to maximal consistent sets of sentences or propositions, i.e., to sets of sentences or propositions which are consistent and to which no further sentence or proposition could be added without their becoming inconsistent. These sets, sometimes termed complete novels or world stories, will henceforth be referred to as (singular MCS). This reduction has been advocated by Robert Adams [1] 225 ff., Thomas Baldwin [2] 60 ff., Frank Jackson [7] 18-19, Richard Jeffrey [8] 196 if., Hans Kamp [9] 34 if. and William Lycan [13] 312 if. Unfortunately, neither these nor any other writers appear to have provided a detailed systematic specification of the reduction. That is, they have not shown how all sentences in a possible worlds semantics which quantify over possible worlds can be adequately replaced by sentences which quantify over MCSS instead. Susan Haack [4] 191 and Lycan [13] 302 have attributed a reductive position to Jaakko Hintikka, who does present a modal semantics in terms of (non-maximal) consistent sets of sentences or sets. However, Hintikka [6] 25-6 explicitly states that the sentences in model sets (partially) describe (rather than replace) possible worlds, and at [6] 60 relies on the notion of possible states of affairs in his explication of the relation of alternativeness between model sets.1 Lycan [13] 302, 312 also attributes a reductive position to Alvin Plantinga, who gives a detailed account of necessity at [16] 44 ff. in which he refers to MCSS of propositions. However, Plantinga also refers to the possible worlds in which these propositions are true, and does not attempt to reduce all of his statements in terms of possible worlds to statements in terms of MCSS, or in any way commit himself to reducing the worlds to MCSS.2 Hence