A decisive difference between our method and Frege's consists in the fact that our concepts, in distinction to Frege's, are independent of the context. Meaning and Necessity (1947)
Frege's pair of concepts (nominatum and sense) is compared with our pair (extension and intension). The two pairs coincide in ordinary (extensional) contexts, but not in oblique (nonextensional) contexts. Meaning and Necessity (1947)