  Mach is part of the empiricist tradition, but he also believed in something like a priori truths. But it is a biologized a priori: what is a priori to an individual organism was a posteriori to its ancestors; not only does the a priori pre-form experience, but the a priori is itself formed from experience. It was simultaneously the contradiction and confirmation of Kantian epistemology. In as much as Kant used the a priori to explain how knowledge is possible, Mach uses the knowledge of the new sciences to explain how an a priori is possible. One more patch of philosophy, it was thought, yielded to science.