 In the philosophy of logic, one of Łukasiewicz’s most deep-seated convictions, one that he shared with the other logicians of the Warsaw School, was that logic has to be extensional, that it is the study of the calculi not of linguistic meanings or psychological judgements but of the truth-values, whether just the classical two or more besides. His view is that sentences denote truth-values, and that logic is the science of such logical values, not of sentences (which is grammar) or of judgements (which is psychology), or of contents expressed by propositions, or of objects in general (ontology). He does not justify this position, but simply accepts and assumes it. As we saw, it has far-reaching consequences for his treatment of modal logic, forcing it to be many-valued.