Kazimierz Twardowski (20/10/1866, Vienna – 11/02/1938, Lvov) is most commonly known as the teacher of great philosophers and the founder of the Lvov-Warsaw School. As a philosopher however, he is primarily remembered for his famous comparison of the contents and objects of various kinds of representations, a comparison that remains enshrined in European thought. In fact, he attained important results in many other branches of philosophy as well. For instance, in (descriptive) ontology, he laid the foundations for the modern theory of formal structure of objects, and he introduced the theoretically fruitful pair of terms, action-product. In epistemology, he developed a profound analysis of the notion and criteria of truth; and he provided a forceful account of the errors underlying relativist theories of truth. In methodology, he drew an explicit distinction between the processes of discovering, systematising, and grounding in science, and he offered accurate descriptions of the nature of psychology and other humanities. In (philosophical) logic, he offered decisive arguments on behalf of the idiogenetic conception of judgement, and he improved the traditional typology of adjectives. These achievements are of significance that is not only historical. Kazimierz Twardowski's work, formulated in plain, precise language, are instructive and inspiring for contemporary students of philosophy.