All logical truth and all truths that logic can warrant must turn upon meaning in the sense of intension. Because logic and the logically certifiable comprise only such facts as are independent of all particular experience and are capable of being known with certainty merely through clear and cogent thinking. The same must hold of any analytic truth: if it is capable of being known by taking thought about it, then it must be independent of meaning in the sense of extension and turn upon meanings only in the sense of intension.
Clarence Lewis (1883–1964), U.S. logician, pragmatist philosopher. repr. in Readings in the Philosophy of Language, pp. 15-42, eds. J. Rosenberg and C. Travis. "The Modes of Meaning," ch. III, pp. 35-70, An Analysis of Knowledge and Valuation (1946).
On the author's view that "extensional logic" is an oxymoron.