In his Introduction to the Science of Law and Morals,¹ Leon Petrażycki (1867–1931) analyzed the concept of adequate theorem, obviously without realizing the fact that similar ideas had been expounded several times before in the history of logic and methodology. My intention is to compare below the texts of his predecessors with his own. I have already done that twice: for the first time very briefly in my paper "On the History of the Concept of Adequate Theory" (in Polish, published in Przeglad Filozoficzny, 1936, in the issue dedicated to the Third Congress of Polish Philosophers), and for the second, much more comprehensively, in a paper bearing the same title and published in Przeglad Filozoficzny, 1937, No. 3. Both papers are included in my collected papers, Reflections on Thinking (in Polish Scientific Publishers: Warsaw, 1958). On this occasion I would like to complete my earlier writings on the subject by quoting the sources.