  According to Ajdukiewicz, natural languages are open and disconnected. By contrast, scientific languages are closed and disconnected. Let L be closed and connected. The set of meanings of L is its conceptual apparatus. If A and A′ are two conceptual apparatuses, they are either identical or mutually non-translatable. Since the acceptance and rejection of sentences is always related to a language L, empirical data do no force us to accept or reject any sentences, because there always remains the possibility of changing a given conceptual apparatus. This is a considerable radicalization of the conventionalism of Poincaré. The difference is the following. For Poincaré, since theoretical principles are conventions, we are free to modify them, but experiential reports are perfectly stable. Ajdukiewicz extended conventionalism to all sentences, because any sentence, no matter whether experiential or theoretical, depends on a conceptual apparatus. It is why Ajdukiewicz called this conventionalism radical.