Extensionalism makes it possible to study logic independently of any theory of meaning. Frege's logic was not fully extensional, however: he developed a theory of meaning in order to have classes uniquely determined. That theory is also flexible enough to allow statements of identity to be at times analytic, at times not. He rejected the traditional empiricist epistemology as an obstacle since it is psychologistic – even though he could not replace it. (A few years after Frege died Popper proposed the first non-psychologistic theory of science.)