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Attention was directed to modal systems in which ‘necessarily α’ is interpreted as ‘α. is and always will be the case’ by Prior in his John Locke Lectures of 1956. The present paper shows that S4.3, the extension of S4 with
is complete with respect to this interpretation when time is taken to be continuous, and that D, the extension of S4.3 with
is complete with respect to this interpretation when time is taken to be discrete. The method employed depends upon the application of an algebraic result of Garrett Birkhoff's to the models for these systems, in the sense of Tarski.
A considerable amount of work on S4.3 and D precedes this paper. The original model with discrete time is given in Prior's  (p. 23, but note the correction in ); that taking time to be continuous yields a weaker system is pointed out by him in . S4.3 and D are studied in  of Dummett and Lemmon, where it is shown that D includes S4.3 and
While in Oxford in 1963, Kripke proved that these were in fact sufficient for D, using semantic tableaux. A decision procedure for S4.3, using Birkhoff's result, is given in my . Dummett conjectured, in a conversation, that taking time to be continuous yielded S4.3. Thus the originality of this paper lies in giving a suitable completeness proof for S4.3, and in the unified algebraic treatment of the systems. It should be emphasised that the credit for first axiomatising D belongs to Kripke.