According to Kuhn, the scientific paradigms preceding and succeeding a paradigm shift are so different that their theories are incommensurable — the new paradigm cannot be proven or disproven by the rules of the old paradigm, and vice versa. (A later interpretation by Kuhn of 'commensurable' versus 'incommensurable' was as a distinction between "languages", namely, that statements in commensurable languages were translatable fully from one to the other, while in "in"commensurable languages, strict translation is not possible.) The paradigm shift does not merely involve the revision or transformation of an individual theory, it changes the way terminology is defined, how the scientists in that field view their subject, and, perhaps most significantly, what questions are regarded as valid, and what rules are used to determine the truth of a particular theory. The new theories were not, as the scientists had previously thought, just extensions of old theories, but were instead completely new world views.