This paper contains a rigorous treatment of the species of implication described in [8] and [9], where it was given the name of connexive implication. A brief historical survey will lay bare its roots in antiquity, and it will be shown that none of the well-known systems of propositional logic serves to formalize it.1 In this paper a new system of ‘connexive’ logic will be presented, the system being shown to be (a) consistent, (b) independent of two-valued logic, (c) Post-complete.

By inequation is meant an expression of the form,

where a, b are terms taken from any first-order formal language. An inequation is consistent iff it cannot be brought to the form,

by substitution.

Briefly, the following things are done in this paper. An informal exposition of the significance of partial truth tables in modal logic is presented. The intuitive concept of a truth tabular sentence connective is examined and made precise. From among a vast assortment of truth tabular connectives, the semantically well-behaved ones (called regular connectives) are singled out for close investigation. The familiar concept of functional completeness is generalized to all sets of regular connectives, and the functional completeness (or incompleteness) of selected sets of connectives is established. Some modal analogues of Sheffer's stroke are presented, i.e., single connectives are introduced which serve to define not only the truth functional connectives but all the regular modal connectives as well. The notion of duality is extended to all regular connectives and a general duality theorem is proved. Lastly, simplified proofs are given of several metatheorems about the system S5.

J. R. Shoenfield conjectured in a talk at the Berkeley Model Theory Symposium (1963) that, if b and d are non-zero recursively enumerable (r.e.) degrees such that b < d then there exists an r.e. degree c such that c < d and b U c = d. G. E. Sacks echoed this conjecture at the end of [3]. In this paper the conjecture is disproved. We construct r.e. degrees b, d such that 0 < b < d and such that for no r.e. degree c is it true that c < d and b U c = d. We are grateful to G. E. Sacks for suggesting this problem.

The purpose of the present paper is to introduce a generalisation of the concept of productive set introduced by Post (2) and studied by Dekker (3) and others. Throughout the paper we shall use small Latin letters to denote both non-negative integers (referred to as numbers) and functions (both partial and total) from numbers to numbers. Sets of numbers will be denoted by small Greek letters and classes of such sets by capital Latin letters. ωη is the range of the ηth partial recursive function.

In the course of a recent paper on modal' extensions of the intuitionist propositional calculus, [1], I made some suggestions as to the relationships between the system MIPC, the intuitionist predicate calculus, and the question of producing a genuine intuitionist concept of modality. This paper may be regarded as a clarification of those rather inaccurate ideas in the light of Kripke's outstanding analysis of the intuitionist predicate calculus, [2]. (I use Kripke's notation and terminology here without explanation — this work is intended to be read in conjunction with [2].) In particular, I shall adapt his interpretation of his modelling to give an account of MIPC in terms of differing mathematical intuitions.

Semantical systems that distinguish between logically true and factually true formulas are well-known from the work of Carnap. The present paper is concerned with extending the formalization of this distinction in two ways. First, we show how to construct syntactical (i.e., logistic) systems that correspond to semantical systems of the type just mentioned. Such a syntactical system for propositional logic is developed in section 3. Similar systems for first-order logic are sketched in section 5. Second, we show how to extend semantical systems that make the logical-factual distinction to languages containing modal connectives. Carnap's work on modal logic conspicuously lacks this feature. Section 4 contains such semantical systems for four well-known modal logics. It also contains a syntactical equivalent of one of these modal semantical systems.

In the first-order language, quantifiers (∀x) and (∃#) are understood “to say” that “for all elements” and “there is at least one element such that …”, respectively. We are interested in changing the interpretation to “for all elements with fewer than κ exceptions” and “there are at least κ elements such that”, respectively, where κ is a cardinal. We call this the κ-interpretation of the quantifiers.1 The first question which presents itself is “What is the relationship between the κ-interpretation and the λ-interpretation?” For instance, is a formula valid under one interpretation also valid in all other interpretations? In the second section, it will be shown that as far as infinite interpretations, i.e. κ-interpretations for infinite cardinals κ, are concerned, the validity of a formula is preserved. Actually, a more general result is obtained there by model theoretic methods.

An effective method for testing arguments consisting of singly quantified expressions has recourse to Parry's trapezoid symbolism.1 The procedure entails representing in that symbolism each premise of the argument and the negation of the conclusion. Variables are disregarded. After clearing away any negated quantifiers, the universal statements, including a denied existential conclusion, are first set down and their conjunction (KU) derived. Existential statements, including a denied universal conclusion, are then listed. The argument is valid if and only if some contradiction appears, whether as some premise empty of lines, as a KU empty of lines, or as some existential statement having no lines in common with the KU.

The equi-cardinality quantifier1 to be used in this article, written as Qx, is characterised by the following semantical rule: A formula QxA(x) is true in a relational system exactly when the cardinality of the set consisting of these elements which make A(x) true is the same as that of the universe. For instance, QxN(x) is true in 〈Rt, N〉 but false in 〈Rl, N〉 where Rt, Rl, and N are the sets of rational numbers, real numbers, and natural numbers, respectively. We notice that in finite domains the equi-cardinality quantifier is the same as the universal quantifier. For this reason, all relational systems considered in the following are assumed infinite.