Thesis beloved of logical atomists, logical positivists, and various kinds of nominalists and reductionists. It says that apparent exceptions to Leibniz's law can be dispensed with; that is, intensions can be reduced to extensions, or roughly, what holds true of objects does not depend on how they are described.
For logical atomism in particular the thesis says that all propositions are truth-functions of certain basic ones (that is, their truth or falsity follows, given the truth or falsity of the basic ones).
An alternative version of the thesis (by Willard Van Orman Quine (1908-2000)) says that only if the above is true can a coherent system of logic be constructed; that is, there is no intensional logic.
W V O Quine, 'Reference and Modality', From a Logical Point of View, 2nd revised edn (1961); reprinted with discussions in L Linsky, ed., Reference and Modality (1971)