The aim of this paper is to present a decision procedure which seems to be as easy to use as other available procedures in quantification theory, but which is considerably stronger than the others, providing a mechanical test for a sub-species of polyadic validity which is very much broader than monadic validity. Of course, a test for polyadic validity in general is out of the question, but the present test's limits, short of polyadic validity, are not known. That is, of all the polyadically valid schemata which have been tested, ncne has failed to yield decisions under this method. The material thus examined includes schemata corresponding to the cases of all absolute metatheorems in ML's Chapters II and III, and five polyadic samples from MeL. The handiness of this test is illustrated by sample applications in § 3 below.

There occurs in Aristotle a thesis which is symbolised in I. M. Bocheński's La Logique de Théophraste, Chapter V, by the formula EMpMNp. We might read this as “It is possible that P (Mp) if and only if (E) it is possible that not P (MNp).” But as Bocheński explains, Aristotle is here using the word “possible” to mean, not merely “not impossible”, but “neither necessary nor impossible”. And if we take “ It is impossible that P” to mean “It is necessary that not P”, Aristotle's thesis seems evidently true. For “It is neither necessary nor impossible that P” (Mp) amounts to “It is not necessary that P, and it is not necessary that not P” (KNSpNSNp, where S is the symbol for “It is necessary that”); and “It is neither necessary nor impossible that not P” (MNp) amounts to “It is not necessary that not P, and it is not necessary that not not P” (KNSNpNSNNp); and this reduces easily to the other by the law of double negation (ENNpp) and the commutativeness of conjunction (EKpqKqp).

Despite the apparently unexceptionable character of this thesis EMpMNp, when properly understood, Bocheński informs us, in paragraph 42 of this chapter, that Łukasiewicz has shown that it “implies a contradiction”. He goes on to explain that what he means by this is that it follows from this thesis, together with the assumption that some propositions are “possible” in the Aristotelian sense, that any proposition whatever is “possible” (in that sense). And this, when one thinks what it means, is a most extraordinary contention.

Halldén, in [1], has recently pointed out that it is highly undesirable, in a system of sentential calculus, for there to exist two formulas α and β such that: (i) α and β contain no variable in common; (ii) neither α nor β is provable; (iii) α ∨ β is provable. We shall call a system unreasonable (in the sense of Halldén) if there exists a pair of formulas α and β having properties (i), (ii), and (iii). Halldén shows (in [1]) that the Lewis systems S1 and S3 are unreasonable in this sense; and that the same is true of any system which is between S1 and S3, as well as of every system which is stronger than S3 but weaker than both S4 and S7. In the present note we shall show that this defect does not occur in S4, nor in S5, nor in any “quasi-normal” extension of S5; we give an example, on the other hand, of an unreasonable system which lies between S4 and S5.

When we speak, in what follows, of a system of modal logic, we shall mean a system having the same class of well-formed formulas as have the various Lewis calculi. Thus the well-formed formulas of a system of modal logic, when written in unabbreviated form, are just those formulas which can be built up from sentential variables by use of the binary connective ‘·’ (conjunction sign), and the two unary connectives ‘˜’ (negation sign) and ‘◇’ (possibility sign). We shall, however, also make use of some of the defined signs of Lewis.

A class A for which there is an infinite progression of classes A1, A2, … (not necessarily all distinct) such that

is said to be groundless. A class which is not groundless is said to be grounded. Let K be the class of all grounded classes.

Let us assume that K is a groundless class. Then there is an infinite progression of classes A1, A2, … such that

Since A1 ϵ K, A1 is a grounded class; since

A1 is also a groundless class. But this is impossible.

Therefore K is a grounded class. Hence K ϵ K, and we have

Therefore K is also a groundless class.

This paradox forms a sort of triplet with the paradox of the class of all non-circular classes and the paradox of the class of all classes which are not n-circular (n a given natural number). The last of the three includes as a special case the paradox of the class of all classes which are not members of themselves (n = 1).

More exactly, a class A1 is circular if there exists some positive integer n and classes A2, A3, …, An such that

For any given positive integer n, a class A1 is n-circular if there are classes A2, …, An, such that

Quite obviously, by arguments similar to the above, we get a paradox of the class of all non-circular classes and a paradox of the class of all classes which are not n-circular, for each positive integer n.

The purpose of this paper is to present a system of arithmetic stronger than those usually employed, and to prove some syntactical theorems concerning it.

We presuppose the lower functional calculus with identity and functions, and we start with three of Peano's axioms.

The other two (0 ϵ N and x ϵ N .⊃. x′ ϵ N) we do not need since our variables are anyhow restricted to natural numbers. Sometimes in the interest of a uniform notation for functions, we write Sx instead of x′.

Next we have two axioms for μ (the smallest number such that) as follows.

A third axiom for μ must wait until we have defined ≤.

Now we introduce the central feature of the system, the following rule of definition.

RD. Let Φ be a previously unused symbol. Then we can introduce it by a pair of definitions of the following form (n ≥ 0),

where F(x1, …, xn) is a wff in which no symbol occurs which was not previously defined (in particular, not Φ), and in which no free variables occur other than x1, …, xn (and possibly not all of these), and G(x1, …, xn, y) is a wff in which no free variables occur other than x1, …, xn, y (and possibly not all of these), and in which no symbol occurs which was not previously defined, except that Φ may occur but only if its last argument is y.

The purpose of the present paper is to dispel such mystery as may be traceable to use of the ill-chosen term ‘ω-inconsistency’ and to a certain application of the term ‘axiom of infinity’. The application of ‘axiom of infinity’ which I have in mind is made by Rosser in his penetrating studies of the system of my New foundations. Let me begin with a synopsis of that system, which I shall call NF.

NF assumes as primitive just the membership connective ‘ϵ’, the truth functions, and quantification over a single style of variables ‘x’, ‘y’, etc. without type distinctions. The axioms, superimposed on standard quantification theory, comprise the axiom of extensionality ‘(x)(x ϵ y · = · x ϵ z) · y ϵ w · ⊃· z ϵ w’ and the axioms of abstraction, these latter being all sentences of the form ‘(∃y)(x)(x ϵ y · = Fx)’ in which the sentence supplanting ‘Fx’ is stratified (i.e., adaptable to type theory by some assignment of types to variables).

It is a well known fact that Zermelo's Axiom of Choice and Zorn's Lemma are equivalent logical assumptions. However, an investigation from the viewpoint of the hierarchy of types reveals a complication in the nature of this equivalence. In a type-theoretical formalism both postulates enter as spectra (ZAα) and (ZLα) of formulas, where ZAα is Zermelo's Axiom and ZLα is Zorn's Lemma stated for variables of a fixed type α. The complication has its origin in the fact that, although the formula ZAα implies ZLα, in order to deduce ZAα it seems to be necessary to assert Zorn's Lemma ZLβ for a type β which is higher than α. In other words, we don't know a proof assuring the equivalence of the two formulas ZAα and ZLα. Therefore the equivalence of Zermelo's Axiom and Zorn's Lemma has to be understood in the sense that the assertion of the formulas ZAα for all types α implies every one of the formulas ZLβ and conversely the assertion of the spectrum of formulas (ZLα) implies every one of the formulas ZAβ.

In § 3 we shall indicate a type β which makes the deduction of ZAα from ZLβ possible. For this purpose a proof by G. Birkhoff [3] is translated into type-theoretical language.

In § 2 it is shown that ZAα implies ZLα. In this proof no use is made of the notion of well-ordering. Specifically, we do not first deduce the Well-Ordering Theorem as it is usually done in proving Zorn's Lemma from Zermelo's Axiom. However, the method is suggested by Zermelo's second proof of the Well-Ordering Theorem [2].

These systems are roughly natural number theory in, respectively, nth order function calculus, for all positive integers n. Each of these systems is expressed in the notation of the theory of types, having variables with type subscripts from 1 to n. Variables of type 1 stand for natural numbers, variables of type 2 stand for classes of natural numbers, etc. Primitive atomic wff's (well-formed formulas) of Tn are those of number theory in variables of type 1, and of the following kind for n > 1: xi ϵ yi+1. Other wff's are formed by truth functions and quantifiers in the usual manner. Quantification theory holds for all the variables of Tn. Tn has the axioms Z1 to Z9, which are, respectively, the nine axioms and axiom schemata for the system Z (natural number theory) on p. 371 of [1]. These axioms and axiom schemata contain only variables of type 1, except for the schemata Z2 and Z9, which are as follows:

where ‘F(x1)’ can be any wff of Tn. Identity is primitive for variables of type 1; for variables of other types it is defined as follows:

In this third and last paper on inner models we consider some of the inherent limitations of the method of using inner models of the type defined in 1.2 for the proof of consistency results for the particular system of set theory under consideration. Roughly speaking this limitation may be described by saying that practically no further consistency results can be obtained by the construction of models satisfying the conditions of theorem 1.5, i.e., conditions 1.31, 1.32, 1.33, 1.51, viz.:

This applies in particular to the ‘complete models’ defined in 1.4. Before going on to a precise statement of these limitations we shall consider now the theorem on which they depend. This is concerned with a particular type of complete model examples of which we call “proper complete models”; they are those complete models which are essentially interior to the universe, those whose classes are sets of the universe constituting a class thereof, i.e., those for which the following proposition is true:

The main theorem of this paper is that the statement that there are no models of this kind can be expressed formally in the same way as the axioms A, B, C and furthermore it can be proved that if the axiom system A, B, C is consistent then so is the system consisting of axioms A, B, C, plus this new hypothesis that there exist no proper complete models. When combined with the axiom ‘V = L’ introduced by Gödel in (1) this new hypothesis yields a system in which any normal complete model which exists has for its universal class V, the universal class of the original system.