  This attitude to logic began to be overtaken by the development of logical semantics, not least at the hands of his own former student Tarski. The turning point came with the publication of Tarski’s essay on the concept of truth in the languages of deductive sciences. This paper was produced in a preliminary version in 1929–30, updated when Gödel’s incompleteness results became known in 1931, and eventually published in Polish in 1933. Leśniewski was known to be opposed to it. There are probably two reasons for this. One is that in his metalogical apparatus Tarski avails himself of set theory. Even though his use is not extensive, it is an incursion of set theory into the metatheory of logic, of which Leśniewski could not but disapprove. The other reason is probably that whereas in earlier parts of the monograph Tarski adheres faithfully to a conception of finite types akin to Leśniewski’s theory of semantic categories, in later parts he distances himself from this as an unwarranted restriction and accepts the propriety of transfinite types.