Śleszyński left Odessa and went to Poland in 1911 where he was appointed as an extraordinary professor at the Jagellonian University of Kraków. We should note that in fact Kraków was at this time in the Austro-Hungarian Empire but, remembering Śleszyński's Polish background, it is fair to say that he was moving to Poland. In 1919 he was promoted from extraordinary professor to become the full Professor of Logic and Mathematics the Jagellonian University. He continued to teach at Kraków until, having reached the age of seventy, he retired in 1924. In fact we note that the university decided not to fill his chair after he retired.
Śleszyński's main work was on continued fractions, least squares and axiomatic proof theory based on mathematical logic. In a paper of 1892, based on his doctoral dissertation, he examined Cauchy's version of the Central Limit Theorem using characteristic function methods, and made several significant improvements and corrections. Because of the work, he is recognised as giving the first rigorous proof of a restricted form of the Central Limit Theorem.
In 1898 Alfred Pringsheim proved that the condition
In  Bednarowski discusses Śleszyński's book O Logice Tradycyjnej Ⓣ published in Kraków in 1921:-
Śleszyński assumes that the part of traditional logic created by Aristotle is a theory of relations which may hold between two classes. He then askes the following question. Having two non-empty classes A and B, what are the possible relations between them so far as having elements in common is concerned? His answer is that between A and B there holds one and only one of five relations which he symbolises by a, b, g, d, e.Śleszyński then represents the five different situations by using Venn diagrams. In a the two classes A and B coincide, in b the class A is properly contained in B, in g the class B is properly contained in A, in d the classes A - B, B - A, and A intersect B are all non-empty, and in the final case e, A and B are disjoint. Śleszyński then argues as follows. First he says that either A and B have common elements or they do not. If they do not then we have the situation e. Next Śleszyński looks at the situation where common elements exist. Either one of A or B contains an element not in the other, or they do not. If they do not, then we have the situation a. There remains the case where either one of A or B contains an element not in the other. If A fails to contain an element not in B we have b. Otherwise A contains an element not in B. If also A contains an element not in B then d otherwise g. Śleszyński also goes on to consider what happens when empty classes are allowed and shows that three further relations occur.
We should mention another interesting work by Śleszyński, namely On the significance of logic for mathematics (Polish) published in 1923. However, despite the interesting publications we have mentioned, Śleszyński did not publish much of his work. This was rectified by a major two-volume publication in the years following his retirement. One of Śleszyński's most famous students at the Jagellonian University of Kraków was Stanisław Zaremba. In 1925 Zaremba, acting as editor, published the first of two volumes of The theory of proof based on Śleszyński's lectures at Kraków. A second volume appeared in 1929. McCall writes in :-
Much indeed can be learned from the rich collection of [Śleszyński's] papers on various subjects in the realm of formal logic, and of mathematical logic and its history ... Introduction to mathematical logic, complete proof, mathematical proof, exposition of the theory of propositions, the Boolean calculus, Grassmann's logic, Schröder's algebra, Poretsky's seven laws, Peano's doctrine, Burali-Forti's doctrine - these are some of the themes pursued in this work, from which I personally have learned a great deal and thanks to which I have got a clear idea of many an unclear thing.We end this brief biography by giving the following quote by Śleszyński:-
The point of civilization is the exchange of ideas. And where is this exchange, if everybody writes and nobody reads?
Article by: J J O'Connor and E F Robertson