In Anderson and Belnap [8] there was developed a semantics for first degree entailments (fde), i.e., entailments A → B between formulas A and B involving only truth-functions (defined in terms of “or” and “not”) and quantifiers. The key ideas were (i) the notion of a frame ⟨P, FIP, I⟩, where P is a set of (intensional) propositions closed under negation and multiple disjunction, I is a domain of individuals, and FIP is the set of functions from I into P; and (ii) the semantic relation of cons (consequence), as obtaining between a set of propositions taken conjunctively, and a set taken disjunctively; and (iii) the notion of an atomic frame, i.e., a frame generated by a set of propositions X closed under negation, such that for any disjoint subclasses Y and Z of X, Y does not bear cons to Z.

In this paper we continue our study, begun in [5], of the connection between ultraproducts and saturated structures. If D is an ultrafilter over a set I, and is a structure (i.e., a model for a first order predicate logic ℒ), the ultrapower of modulo D is denoted by D-prod. The ultrapower is important because it is a method of constructing structures which are elementarily equivalent to a given structure (see Frayne-Morel-Scott [3]). Our ultimate aim is to find out what kinds of structure are ultrapowers of . We made a beginning in [5] by proving that, assuming the generalized continuum hypothesis (GCH), for each cardinal α there is an ultrafilter D over a set of power α such that for all structures , D-prod is α+-saturated.

It is shown in [1] that an ultraproduct of finite sets can be of arbitrarily large cardinality, but if it is infinite then it must have at least the power of the continuum. In this paper we shall take a closer look at the cardinality of ultraproducts of finite sets. Our results were announced without proof in [5]. A discussion of the cardinality of ultraproducts of infinite sets, and another theorem about ultraproducts of finite sets, can be found in [3].

In [1, pp. 82–84] L. Åqvist considers a modal system which he calls S3.5 obtained by adding to S3 the axiom ∼□p⊃□∼□p. This system becomes S5 when the rule ├A→├□A is added to it. S3.5 is put forward to stand to S5 as S3 stands to S4 and S2 to T. In this note we show how a natural extension of the modelling for S3 in [2] can give a suitable semantics for S3.5.1

An S3 model2 is an ordered quadruple (GKRφ) where Κ is a set, G ε K and R is transitive and quasi-reflexive over K.

In the literature, a type Γ is a set of formulas ϕ(ν0) in the one free variable ν0. A model M realizes Γ if there is an element a of M such that

M ╞ ϕ [a] for all ϕ ∈ Γ.

A model M omits Γ if no element of M realizes Γ. Evidently, the two statements:

(*) M realizes Γ

and

(**) M omits Γ

The following describes a calculus of sequents (SLK) patterned after the calculus LK of Gentzen in [1] but restricted to the formulas of propositional logic and modified so that the only connective is the Sheffer stroke. The ‘elimination theorem’ (Hauptsatz) is proved for SLK and a decision procedure is specified for determining whether a given formula in stroke notation is tautologous. In addition, SLK is proved consistent and complete. Subsequent remarks indicate briefly how the calculus can be modified so as to employ the dagger as the only connective.

The presence of a rule for existential instantiation (EI) in a system of natural deduction often causes some difficulties, in particular, when it comes to formulate necessary restrictions on the rule for universal generalization (UG). A system containing rules for EI and UG that avoided Quine's rather cumbersome restrictions on these rules was formulated by Copi [2], but the system was found to be inconveniently restrictive. A less restrictive system was therefore suggested by Copi [3]. Also that system forces some deductions to be unnecessarily long as is shown in Prawitz [5, Appendix C, p. 104], where a way to liberalize Copi's restriction on UG is suggested (p. 105). However, the system suggested by Copi [3] is also incorrect (i.e. unsound) as has recently been shown by Parry [4] in this Journal.

Student. I've been studying the logic of classes, and can't see how any class can be included in its own complement.

Teacher. Surely you've heard of the null class, which is included in every class, and hence in its own complement?

Student. That also strikes me as somehow implausible. How do you define ‘inclusion’?

Teacher. In the usual way. a is included in b if and only if the intersection of a with the complement of b is null.

This paper answers affirmatively the open question of Massey [1] concerning the existence of binary connectives functionally complete by themselves in two-valued truth tabular logic, i.e. in the modal theory S5. Since {∼, ⊃, ◊} is a functionally complete set of connectives (Massey [1, § 4]), the following definitions show that the binary operator ф, the semantics of which is given below, is functionally complete by itself:

It is left to the reader to verify, by means of complete sets of truth tables (see Massey [1, §§ 1 and 3]), that the foregoing definitions are correct.

1.1. The aim of this paper is the construction of a demonstrably consistent system of set theory that (1) contains roughly the same amount of mathematics as the writer's system K′ [3], including a theory of continuous functions of real numbers, and (2) provides a way for expressing in the object language various propositions which, in the case of K′, could be expressed only in the metalanguage, for example, general propositions about all real numbers. It was not originally intended that the desired system should be a modal logic, but the modal character of the system appears to be a natural outgrowth of the way it is constructed. A detailed treatment of the natural, rational, and real numbers is left for a subsequent paper.