The Polish philosopher Kazimierz Twardowski, founder of the modern Polish School of philosophy (Lukasiewicz, Ajdukiewicz, Lesniewski and Ingarden count as his immediate successors), published this now extremely rare book in 1894. It represents a milestone in the history of the Brentano school and therefore of Austrian philosophical history as a whole. Twardowski's return to Bolzano is of decisive importance to Husserl's thinking. It re-establishes the platonistic tradition of Austrian philosophy which influenced even the early Wittgenstein. Rudolf Haller's introductory essay provides a systematic interpretation of Twardowski's work, as well as a description of how it cane to be written and how it was received. Of interest to: Philosophers, logicians, historians, psychologists