Frederic B. Fitch. A further consistent extension of basic logic, The journal of symbolic logic. vol. 14 (1949), pp. 209–218.

1950 ◽  
Vol 15 (3) ◽  
pp. 219-220
Author(s):  
S. C. Kleene
Keyword(s):  
Symbolic Logic ◽  
Basic Logic ◽  
Consistent Extension

An extensional variety of extended basic logic

1958 ◽  
Vol 23 (1) ◽  
pp. 13-21 ◽  
Author(s):  
Frederic B. Fitch
Keyword(s):  
Mathematical Analysis ◽  
Symbolic Logic ◽  
Basic Logic ◽  
Natural Numbers ◽  
Strong Principle ◽  
Theory Of Numbers

The system K′ of “extended basic logic” lacks a principle of extensionality. In this paper a system KE′ will be presented which is like K′ in many respects but which does possess a fairly strong principle of extensionality by way of rule 6.37 below. It will be shown that KE′ is free from contradiction. KE′ is especially well suited for formalizing the theory of numbers presented in my paper, On natural numbers, integers, and rationals. The methods used there can be applied even more directly here because of the freedom of KE′ from type restrictions, but the details of such a derivation of a portion of mathematics will not be presented in this paper. It is evident, moreover, that KE′ contains at least as much of mathematical analysis as does K′, and perhaps considerably more. The method of carrying out proofs in KE′ is closely similar to that used in my book Symbolic logic, and could be expressed in similar notation.

A further consistent extension of basic logic

1950 ◽  
Vol 14 (4) ◽  
pp. 209-218 ◽  
Author(s):  
Frederic B. Fitch
Keyword(s):  
Special Kind ◽  
Basic Logic ◽  
Real Numbers ◽  
Consistent Theory ◽  
The Real ◽  
Recursively Enumerable ◽  
Consistent Extension ◽  
Universal Quantification ◽  
Excluded Middle ◽  
Additional Assumptions

1.1. In two previous papers a consistent theory of real numbers has been outlined by the author, using a system K′. This latter system is an extension of a system K, which is “basic” in the sense that every finitary (recursively enumerable) subclass of its well-formed expressions is in a certain sense represented in it. The system L described below is a further extension of K. The system K′ lacks two important features possessed by L: a symbol for a special kind of implication (or “conditionality”) and a symbol for the modal concept “necessity.” The presence of the implication symbol, and the additional assumptions that go with it, make available in L various kinds of restricted universal quantification not available in K′, for example, universal quantification restricted to the real numbers of the author's theory of real numbers.1.2. If ‘~[a & ~a]’ is a theorem of L, then the proposition expressed by ‘a’ may be said to L-satisfy the principle of excluded middle. I t is always the case that ‘a’ L-satisfies the principle of excluded middle (or rather that the proposition expressed by ‘a’ does so) if and only if ‘a’ or ‘~a’ is a theorem of L. An example of a proposition that does not L-satisfy the principle of excluded middle is that expressed by ‘’, namely the proposition that asserts that the class of classes that are not members of themselves is a member of itself.

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