  In “An Attempt at a Proof of the Ontological Principle of Contradiction” (1912) Leśniewski argues that ‘No object is able to be both \(B\) and not\(-B\)’ is true but disagrees with the equivalence (now generally accepted) that ‘No \(A\) is \(B\)’ is equivalent to ‘If something is \(A\) then it is not \(B\)’. The article is written in criticism of some ideas of Łukasiewicz and it afforded the occasion for the first personal meeting of the two later colleagues. Łukasiewicz described the meeting decades later in a poignant diary entry for 9 May 1949: