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Some Completeness Results for Modal Predicate Calculi

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Philosophical Problems in Logic

Part of the book series: Synthese Library ((SYLI,volume 29))


Two systems of first-order predicate calculus with identity, one of them with definite descriptions, will be formulated in this paper along with semantic interpretations, and then shown strongly complete by methods similar to those of Henkin [3]. These systems, Q1 and Q3,1 are generalizations of the systems presented in Kripke [6] and [8], respectively.2 An informal and philosophical account of Q1 and Q3 can be found in [11], together with a historical note concerning the development of the systems and their interpretation.

The research leading to this paper was supported under National Science Foundation grant GS-1567. I am indebted to Professor Nino Cocchiarella for comments on an earlier draft of this paper.

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  1. The system Q2 is discussed in Thomason [13], where a definition of Q2-validity is given. Prof. David Kaplan has informed me (in a private communication, April, 1967) that the notion of Q2-validity cannot be recursively axiomatized.

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  2. The system Q3 is a generalization of the version of modal predicate calculus described in Kripke [8], which treats only closed formulas, and hence gives no account of individual constants or definite descriptions. On the other hand, the system Q1S5 of modal predicate calculus based on an S5-type modality and on a Ql-type theory of quantification and identity is the system proved semantically complete in Kripke [6]. In this case our generalization of Kripke’s results consists in allowing for sorts of modality other than S5, and in proving strong rather than weak completeness.

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  3. We will use dots in the usual way in place of parentheses; see Church [1], pp. 74–80.

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  4. This proof requires that the morphology (i.e. the set of formulas of the morphology) be denumerable.

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  5. See the articles of Kripke, especially [7] and [8], for an intuitive account of this semantics. Another discussion of this sort may be found in Thomason [13].

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  6. The requirement that the domains be nonempty is easily lifted; in this case, one must also drop A6’ from the system Q3.

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  7. This is the only place in the proof of semantic completeness which must be changed to adjust the argument to kinds of modality other than S4.

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  8. The rules R4—R7 are needed for the proof of semantic completeness of Q3. At present, it is not known whether these rules are redundant.

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  9. We will use individual variables for instantiation rather than individual constants, as is usual in versions of Henkin’s proof and as we ourselves have done in the case of Ql. The underlying reason for this is semantic; in Q3, individual constants and individual variables are wholly different. Whereas variables range over things (i.e. things-as- identified-across worlds), constants need not be assigned one thing; they may name different things in different worlds.

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  10. The proof we give of this lemma makes use of our assumption that the morphology is denumerable.

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  11. This can be made more precise by adding to the definition of a Q3-interpretation on < K, R, D, D’ > an auxiliary function which selects for each definite description 1 x A and member α of K an element of D—D α a to be assigned to 1 x A in case the uniqueexistence condition fails.

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  12. Our account of descriptions differs only in minor respects from the theory given in van Fraassen and Lambert [14] of the system FD.

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  13. A version of T4 is established in Cocchiarella [2] by means of semantic tableaux, for a system of modal predicate calculus with tense-operators. When a connective corresponding to necessity is defined in terms of these operators a system equivalent to our Q3 is obtained. Cocchiarella later proved a compactness theorem for his system, which together with his weak completeness theorem yields a result implying our T3. This, however, has not yet appeared in print.

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  14. The results appearing in the present paper are, to my knowledge, the first Henkinstyle completeness proofs for systems such as Q3. As I see it, the principal advantage of these results is their flexibility. They are very easy to adapt to other systems, especially to ones obtained by extending the language of Q3. For an example of such an application, see Stalnaker and Thomason [11].

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© 1970 D. Reidel Publishing Company, Dordrecht, Holland

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Thomason, R.H. (1970). Some Completeness Results for Modal Predicate Calculi. In: Lambert, K. (eds) Philosophical Problems in Logic. Synthese Library, vol 29. Springer, Dordrecht.

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