from the Writings of

Alfred Jules Ayer

On the Analyticity of Mathematics

Philosophy is a Department of Logic

On the Analyticity of Mathematics

A point which is not sufficiently brought out by Russell, if indeed it is recognised by him at all, is that every logical proposition is valid in its own right*1. Its validity does not depend upon its being incorporated in a system, and deduced from certain propositions which are taken as self-evident. The construction of systems of logic is useful as a means of discovering and certifying analytic propositions, but it is not in principle essential even for this purpose. For it is possible to conceive of a symbolism in which every analytic proposition could be seen to be analytic in virtue of its form alone. The fact that the validity of an analytic proposition in no way depends on its being deducible from other analytic propositions is our justification for disregarding the question whether the propositions of mathematics are reducible to propositions of formal logic, in the way that Russell supposed (1919, chap.2). For even if it is the case that the definition of a cardinal number as a class of classes similar to a given class is circular, and it is not possible to reduce mathematical notions to purely logical notions, it will still remain true that the propositions of mathematics are analytic propositions. They will form a special class of analytic propositions, containing special terms, but they will be none the less analytic for that. For the criterion of an analytic proposition is that its validity should follow simply from the definition of the terms contained in it, and this condition is fulfilled by the propositions of pure mathematics.
Language Truth and Logic, [Ayer36] (Ch.4 p.108)

philosophy is a department of logic

In other words, the propositions of philosophy are not factual, but linguistic in character - that is, they do not describe the behaviour of physical, or even mental, objects; they express definitions, or the formal consequences of definitions. Accordingly we may say that philosophy is a department of logic*2. For we will see that the characteristic mark of a purely logical enquiry, is that it is concerned with the formal consequences of our definitions and not with questions of empirical fact.
Language Truth and Logic, [Ayer36] (Ch.2 p.76)  

*1 It appears that this first point is made by Russell, On Defining Logic.

*2 See also Russell, Philosophy as Logic.

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