Quine's strategy for defining analyticity adopted the following line of reasoning. His proposal is that analyticity can be defined in terms of cognitive synonymy which in turn can be defined in terms of interchangeability salva veritate. Thus, two expressions are cognitively synonymous if they are interchangeable and this explains their reason for being analytic. To elaborate, 'A' and 'B' are cognitively synonymous only if 'A' is substitutable for 'B'. From this it follows that if 'A' is cognitively synonymous with 'B' then 'A is B is analytic'.

Crucial to this definition is the notion of interchangeability which fits well in the context of extensional languages. To say, 'A' is extensionally equivalent to 'B' implies 'A' is substitutable everywhere for 'B' and vice versa. This is how the notion of interchangeability is formulated in extensional languages. But the difficulty with this formulation is that it does not satisfy the requirement of positivist epistemology. A definition of this sort cannot support the basic epistemological distinction that positivism makes between two notions of truth, namely; truth due to meaning and truth due to fact. The reason is that within an extensional language we cannot make such a distinction. From the point of view of extensional language, the sentence, 'All bachelors are unmarried' and the sentence 'All chordates are renates' may be viewed as similar because they are true and the reason for their being true is same. They are true on the ground of 'accidental matters of fact', as Quine puts it. The notion of interchangeability is thus defined solely in relation to the world of facts and this is the only notion of interchangeability that an extensional language allows. But this will miserably fail to account for the distinction that the positivist theory of knowledge requires, i.e. the statements that are true by meaning and the statements that are true due to the way facts are organized in the world. It is this epistemological distinction that the notion of interchangeability defined in terms of extensional equivalence is unable to provide.