1. This distinction is closely related to that between radical concepts and L-concepts which I made inIntroduction to Semantics. The contrast between extension and intension is the basis of the semantical method which I developed inMeaning and Necessity. Quine calls the two theories “theory of reference” and “theory of meaning,” respectively.
2. R. Carnap, “Meaning Postulates,”Philosophical Studies, 3:65–73 (1952).
3. W. V. Quine, From aLogical Point of View: Nine Logico-Philosophical Essays (1953). For his criticism of intension concepts see especially Essays II (“Two Dogmas of Empiricism,” first published in 1951), III, and VII.
4. M. White, “The Analytic and the Synthetic: An Untenable Dualism” in Sidney Hook, ed.,John Dewey: Philosopher of Science and Freedom, 1950, pp. 316–30.
5. Some philosophers have indeed defined the intension of a predicate (or a concept closely related to it) as the class of the possible objects falling under it. For example, C. I. Lewis defines: “The comprehension of a term is the classification of all consistently thinkable things to which the term would correctly apply” (“The Modes of Meaning,”Philosophy and Phenomenological Research, 4:236–50 (1944)). I prefer to apply modalities like possibility not to objects but only to intensions, especially to propositions or to properties (kinds). (CompareMeaning and Necessity, pp. 66f.) To speak of a possible case means to speak of a kind of objects which is possibly non-empty.