  The turning point in this period came when in 1911 Leśniewski read Łukasiewicz’s groundbreaking 1910 monograph O zasadzie sprzeczności u Arystotelesa [On the Principle of Contradiction in Aristotle], a radical rethink of the status of the principle of contradiction in the light of modern logic. That work contained an appendix in which there was a short exposition of modern symbolic logic, in the notation of Couturat, and in Chapter 18 a brief discussion of Russell’s antinomy of the set of sets which are not members of themselves. Leśniewski initially thought the Russell Antinomy was easy to fix—and while trying to fix it missed a once-a-day train when changing stations in Russia. He spent the next years perfecting his solution, and indeed the rest of his life was devoted to providing a rigorous, antinomy-free foundation for mathematics which avoided both the sloppiness of Principia and what he considered the fiction of standard set theory.