For the formulation of the remaining axioms we need the notions of a function and of a one-to-one correspondence.

We define a function to be a class of pairs in which different elements always have different first members; or, in other words, a class F of pairs such that, to every element a of its domain there is a unique element b of its converse domain determined by the condition 〈a, b〉ηF. We shall call the set b so determined the value of F for a, and denote it (following the mathematical usage) by F(a).

A set which represents a function—i.e., a set of pairs in which different elements always have different first members—will be called a functional set.

If b is the value of the function F for a, we shall say that F assigns the set b to the set a; and if a functional set f represents F, we shall say also that f assigns the set b to the set a.

A class of pairs will be called a one-to-one correspondence if both it and its converse class are functions. We shall say that there exists a one-to-one correspondence between the classes A and B (or of A to B) if A and B are domain and converse domain of a one-to-one correspondence. Likewise we shall say that there exists a one-to-one correspondence between the sets a and b (or of a to b) if a and b respectively represent the domain and the converse domain of a one-to-one correspondence. In the same fashion we speak of a one-to-one correspondence between a class and a set, or a set and a class.

The purpose of this paper is to put on record some theorems relating to improvements in the primitive frame of combinatory logic. These improvements were, for the most part, suggested by the work of Rosser, who formulated a weakened system of combinatory logic in which the rules had a simple character not possessed by those of the original system. In the latter the rules B, C, W, K were in reality axiom-schemes, and their postulation amounted to assuming infinitely many axioms. Rosser had rules of procedure such that no propositions were deducible from them except in combination with axioms (or previously proved propositions); moreover, the conclusion of each rule was uniquely determined by the premises. He also eliminated equality as a primitive term, defining it (essentially) according to the traditional method. This paper shows that these and related advantages apply to certain formulations of the full system of combinatory logic, so far as it concerns the theory of combinators.

The method of procedure is as follows. Instead of setting up a primitive frame at the start and then deriving its properties, I begin (after some preliminary explanations in §2) by stating in §3 the properties which it is desired to establish. The next few sections are devoted to the formulation and proof of certain general theorems concerning possible bases for the system of §3. These theorems are, perhaps, more general than is necessary for the immediate purpose, but they are of some interest on their own account. A formulation in terms of the primitives of original system (i.e., B, C, W, and K), which is of the same general type as Rosser's formulation, is obtained at the end of §6. In §7 are discussed the changes in this formulation which are sufficient in order to base it on the primitive combinators S and K of Schönfinkel.

The logical theory which is called the calculus of (binary) relations, and which will constitute the subject of this paper, has had a strange and rather capricious line of historical development. Although some scattered remarks regarding the concept of relations are to be found already in the writings of medieval logicians, it is only within the last hundred years that this topic has become the subject of systematic investigation. The first beginnings of the contemporary theory of relations are to be found in the writings of A. De Morgan, who carried out extensive investigations in this domain in the fifties of the Nineteenth Century. De Morgan clearly realized the inadequacy of traditional logic for the expression and justification, not merely of the more intricate arguments of mathematics and the sciences, but even of simple arguments occurring in every-day life; witness his famous aphorism, that all the logic of Aristotle does not permit us, from the fact that a horse is an animal, to conclude that the head of a horse is the head of an animal. In his effort to break the bonds of traditional logic and to expand the limits of logical inquiry, he directed his attention to the general concept of relations and fully recognized its significance. Nevertheless, De Morgan cannot be regarded as the creator of the modern theory of relations, since he did not possess an adequate apparatus for treating the subject in which he was interested, and was apparently unable to create such an apparatus. His investigations on relations show a lack of clarity and rigor which perhaps accounts for the neglect into which they fell in the following years.

In this paper I shall give a solution of the decision problem for the Lewis systems S2 and S4; i.e., I shall establish a constructive method for deciding whether an arbitrary given sentence of one of these systems is provable. The method is laborious to apply, since, in order to decide by means of it whether a given sentence is provable, it is necessary to construct a (usually very large) finite matrix. The argument will perhaps be of general interest, however, because it does not seem to depend too closely on the special features of these particular systems, so that it may be possible to apply it in order to solve the decision problem for other such systems.

Section II presents the decision method for S2, and Section III for S4. In Section IV, I shall establish a certain correspondence between S4 and topology, which will provide a solution for a decision problem in topology; this correspondence also enables us to settle a previously unsolved problem with regard to the Lewis systems.

In treating of this system, I shall use the notation of Lewis, with the single exception that I shall use the symbol “≣”, instead of Lewis's symbol “=”, for strict equivalence. I shall use the symbol “=” to denote identity. Thus “p≣q” is a formula of S2, while “x=y” is the statement asserting that x and y are identical. I shall refer to the rules, primitive sentences and theorems of S2 by the names and numbers used by Lewis. Whenever a theorem stated by Lewis involves the symbol “=”, I shall of course suppose that this symbol has been replaced throughout by “≣”: thus, for example, I shall take 19.82 to be “(◇p∨◇q) ≣ ◇(p∨q)”, instead of “(◇p∨◇q) ≣ ◇(p∨q)”.

The purpose of this paper is to suggest two alternatives to Quine's definition of closure. These new definitions have two advantages over Quine's definition, and they probably are the simplest definitions having both advantages. The two advantages are:

(1). Principle *101 becomes superfluous and may be dropped from Quine's set of principles for quantification. (In the case of my second definition, however, the dropping of *101 must be balanced by a slight change in *104.)

(2). Closure is made independent of the alphabetical order of variables.

The second of these advantages turns on the fact that the “alphabetical order” possessed by variables in virtue of their respective positions in the alphabet (or arbitrarily assigned to them) is a mere convention and not of genuine logical significance. It seems therefore desirable to consider some alternatives to Quine's definition of closure, since according to his definition the closure of a given formula will be one statement or another, depending upon whether or not one letter of the alphabet is alphabetically prior to a certain other letter. It is interesting that the removal of this minor artificiality also enables us to dispense with *101.

According to Quine, the closure of a formula containing n free variables is obtained by prefixing to it in alphabetical order the n universal quantifiers formed from these variables by enclosing each in a pair of parentheses. (If n = 0 the formula is its own closure and is a “statement” rather than a “matrix.”) Thus the statement ‘(x)(y)(z)(xϵy ▪ yϵz)’ would be the closure of ‘xϵy ▪ yϵz’, but ‘(z)(x)(y)(xϵy ▪ yϵz)’ would not be its closure. Now there is no reason why ‘(z)(x)(y)(xϵy ▪ yϵz)’ or (y)(z)(x)(xϵy ▪ yϵz) and so on, could not just as well be regarded as “the” closure of ‘(xϵy ▪ yϵz’ as ‘(x)(y)(z)(xϵy ▪ yϵz)’. I therefore propose to allow to each formula not merely one closure, but as many closures as can be obtained by permuting in various ways the n prefixed universal quantifiers. In this way alphabetical order becomes irrelevant and no preference is given to one order of prefixed quantifiers in contrast to other orders which seem equally good. This constitutes my first redefinition of closure.

The Ladd-Franklin “antilogism” provides a very convenient procedure for testing syllogistic arguments, especially when they are expressed algebraically. It is well known, however, that the antilogism test indicates the invalidity of certain moods which the traditional logic regards as valid. These are the moods in which a particular conclusion is inferred from two universal premises. It is the aim of this paper to show that the antilogism can be generalized in such a way as to cover these moods, and also to be applicable to certain forms of “immediate inference” and “sorites,” as these designations have been traditionally employed.

We shall find that when generalized in this fashion there are two principal kinds of antilogisms: (1) those which include an inequation of the form ab≠0, and (2) those which include an inequation of the form a≠0.

Definition. An antilogism is a group of propositions so constructed that the contradictory of any one of them is implied by the logical product of the others.

The consistency of the theory of combinators, when based on the combinatory axioms, the rules of equality, and the equations

was proved in my thesis; likewise some theorems expressing certain completeness properties of this theory of combinatory equivalence formed the principal subject of that thesis and a succeeding paper. Further theorems of this sort were later demonstrated by Rosser; his theorems are, in some cases, more powerful than those just mentioned, but they apply to a weakened system. The purpose of this note is to discuss the extension of these theorems to the case where the whole of the theory of combinators is taken into account, and the rules for equality are derived from more fundamental considerations.

The paper is based on a formulation of the theory of combinators contained in a previous paper, acquaintance with which is presupposed.

A key theorem in investigations of this character is a theorem proved in 1936 by Church and Rosser. It is necessary to give a brief account of this theorem together with a generalization of it.

The theorem concerns a certain formalism, which I shall here call the λ-formalism. The terms of this formalism, which I shall call λ-terms, are constituted as follows. For primitive terms we have an infinite set of variables, together with certain constants, the nature of which is arbitrary. Further terms are formed by two operations: a binary operation (analogous to application in the theory of combinators) which combines two λ-terms and to form a new λ-term here symbolized by ; and another binary operation combining a variable x and a term containing x as free variable to form a term .

We refer to the axioms in Quine's book, Mathematical logic, New York, 1940.

To prove the independence of *200, give xϵ α the truth value F in all cases and give (x)ϕ the same truth value as ϕ. Then clearly all formulas derivable from the other axioms besides *200 have the value T, whereas from *200 one can derive (∃x)(∃α)(xϵ α) which has the value F. This method of proving independence amounts to taking for a model a universe consisting of the single object Λ.

For *201 we prove a contingent independence. That is, we prove that if Quine's system is consistent, then *201 is independent. The line of argument is the following. Suppose *201 can be derived from the other axioms. Let us replace xϵ α by throughout all the axioms. Then what *201 becomes can be derived from what the other axioms become. However what *201 becomes will lead to a contradiction in Quine's system whereas the rules which the other axioms become are valid in Quine's system.

We now get down to technical details. Let us refer to the replacement of xϵ α by throughout an expression as an r replacement. Denote the result of performing an r replacement on ϕ by ϕr. Let Wα denote

Then

Note that if x and y are variables, then by D10,

In his Mathematical logic, Quine adopts as axioms of quantification all statements specified by the first five of the six metatheorems listed below. The sixth serves as a rule of inference, and is the only primitive rule of inference used. The metatheorems in question are as follows:

The theme of the present paper is that *101 may be eliminated by means of an otherwise trivial shift in the meanings of *100, *102, *103, and *104.

In the statement of the above metatheorems and throughout the paper as a whole, we deviate from Quine's notational conventions only in using double quotes rather than corners and in using English letters rather than Greek ones as syntactical variables. Accordingly, ‘a’, ‘b’, and ‘d’ ambiguously denote variables ‘x’, ‘y’, ‘z’, and so on. The letters ‘p’ and ‘q’ ambiguously denote formulae, i.e. statements, and expressions which become statements when their contained free variables are supplanted by constants. Since we replace Quine's corners with double quotes, “(a)p” (for instance) is not the result of prefixing a parenthesized occurrence of the first letter of the alphabet to an occurrence of the sixteenth: “(a)p” is rather the result of prefixing any parenthesized variable a to any formula p.

Following Quine, we define the closure of p as p itself when p is a statement and as “(a1)(a2)…(an)p” where a1, a2, …, an are the sole variables free in p and where each ai is alphabetically prior to the corresponding ai+1.