In this paper a semantics and logic of conditional necessity is developed as the basis of a logic of indicative and subjunctive conditionals and of causal sentences. It is argued, against E. Adams and D. Lewis, that these three types of statements differ only in presupposition. In "Counterfactuals" (1973) David Lewis has developed a logical system VC for counterfactuals. For the normal cases of counterfactuals, in which the antecedent is false, VC can be replaced by VW, a system based on weak

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... stead of strong centering. In this paper I shall try to show that this system can also be applied to indicative conditionals and can generally be used for a comprehensive and unified treatment of the logic of conditionals. The main obstacles in generalizing Lewis' analysis of counterfactuals to include indicative conditionals are, first, that the intuitive background he provides for his semantics favours strong centering, and, second, an argument by Ernest W. Adams in (1970) to the effect that "subjunctive and indicative conditionals are... logically distinct species" so that the truth-conditions for the former cannot be derived from those for the latter by adding suitable presuppositions. Our first step will be a reconsideration of that argument. Cf. also Lewis (1973), p. 71. Cf. Goodman (1965), pp. 14 seq. 3 Cf. for instance Kutschera (1974 a). 4 We shall not discuss the difference between subjective and objective presuppositions here. 5 Cf. Lewis (1973), pp. 48 seq.