  Paraphrasing a theory into classical logic imposes extensionality on it: a predicate may be replaced by a co-extensive predicate without change of truth-value of the containing sentence; likewise an embedded sentence by a sentence of the same  truth-value.[3] Extensionality imposes quite severe restrictions. It requires, for example, that attributions of belief, and other propositional attitudes, be regimented into a form quite different from that which they may appear to have. (For this reason, the commitment to extensionality is quite controversial; see Føllesdal 1986 and Kaplan 1968, 1986.) So one might suppose that Quine accepts extensionality reluctantly, as the price to be paid for the advantages of the use of logic in regimenting theory. Such is not his attitude, however. To the contrary, he thinks of the clarity that extensionality brings as a great advantage, and of theories that lack it as not fully comprehensible: “I find extensionality necessary, indeed, though not sufficient, for my full understanding of a theory.” (1995, 90f.)