 Łukasiewicz introduces the project of his monograph, a critical investigation of the legitimacy of the Principle of Contradiction (PC) as variously formulated by Aristotle, in the context of its critique by Hegel and the opportunity to re-examine the PC in the light of the development of mathematical logic from Boole to Russell. Łukasiewicz’s sources for the post-Hegelian discussion of the ‘logical question’ are Ueberweg, Trendelenburg and Sigwart. A more local background was probably Twardowski’s account of the absolute and timeless nature of truth.