  In the middle of the 1930s Ajdukiewicz changed his view. He came to the conclusion that closed and connected languages are fictions. He was influenced by the semantic ideas of Tarski. Tarski also argued that, contrary to Ajdukiewicz’s hopes, the invariance of meaning-rules over permutations of expressions influences their meaning-relations. Gradually, Ajdukiewicz developed the program of semantic epistemology, mainly directed toward the defence of realism against various forms of idealism. In particular, he criticized Rickert’s transcendental idealism and Berkeley’s subjectivism. For Rickert, reality is only a correlate of the Transcendental Subject. Now the Transcendental Subject can be identified with the set T of true propositions obtainable on the basis of axiomatic and deductive rules. However, due to incompleteness phenomena, T cannot be generated in this way. For Ajdukiewicz, that was a justification that transcendental idealism fails. Ajdukiewicz compared the language used by Berkeley to the language of syntax, because the former reduces relations of the mind to its objects to relations between thoughts. On the other hand, the ordinary way of speaking about objects employs semantic relations. Berkeley’s claim esse = percipi is similar to an attempt to define semantics in a purely syntactic language. However, due to Tarski’s results about the relation between syntax and semantics, this is impossible. Finally, Ajdukiewicz argued that any idealistic language is understandable only if it is associated with a realistic language. Hence, any attempt to consider idealistic language as self-sufficient cannot be successful.