Popper held that rationality is not restricted to the realm of empirical or scientific theories, but that it is merely a special case of the general method of criticism, the method of finding and eliminating contradictions in knowledge without ad-hoc-measures. According to this view, rational discussion about metaphysical ideas, about moral values and even about purposes is possible. Popper's student W.W. Bartley III tried to radicalise this idea and made the controversial claim that not only can criticism go beyond empirical knowledge, but that everything can be rationally criticised.

Answer this question, if possible (if impossible, reply "unanswerable"): What type of knowledge is not the only sphere of rational criticism, according to Popper's student W.W. Bartley III?
empirical