 Bolzano was a forerunner of important theories and ideas in various areas of knowledge. Many of Bolzano’s ideas had to be discovered anew since Bolzano’s preparatory work remained unknown in many cases and has come to light only from historical research. Superficially speaking, it seems as if Bolzano had hardly any direct influence at all in the proper sense on the development of the sciences, although he accomplished significant research. This may of course in large part be due to the special circumstances under which Bolzano had to work and publish. His writings thus often appeared anonymously and were for this reason often generally unknown — at least not known under his name. Although Bolzano did not directly influence modern developments — in the area of logic, for instance —, he obviously did achieve a very tangible “indirect” and subliminal effect. For Bolzano’s views were familiar to very many who passed them on, partly with the mention of his name and partly without any such mention. Thus, Bolzano’s philosophy has not remained completely ineffectual: We know that Twardowski was well familiar with Bolzano (Twardowski 1894), and we know that Łukasiewicz was well acquainted with Bolzano’s method of idea-variation, as a section in his book on Logical Foundations of Probability Theory (Łukasiewicz 1913, § 24) shows. It is to be suspected that the whole famous Polish school of logic was thereby indirectly influenced by Bolzano without knowing exactly the places where this influence was exerted, and without Bolzano being mentioned in a way which would have been appropriate. Franz Brentano had studied with particular interest, as he himself confessed, the Paradoxes of the Infinite; he was not, however, as well familiar with Bolzano’s philosophy as one often suspected. Nevertheless, it seems improbable that this familiarity with Bolzano, as little as is was, could have remained without any effect on his pupils such as Marty, Meinong, and Stumpf, although some of whom were “outcasts” from the orthodox Brentanian circle. Bolzano’s influence on Husserl, however, is well-known, and Husserl himself made no secret of it.