 In his ‘Self-Portrait’ Twardowski also remarks that it was the question of the nature of concepts that brought him to Content and Object. The issue ensued from Twardowski's research on a particular concept in his doctoral dissertation, Descartes' concept of clear and distinct perception. Since concepts are a species of presentations, Twardowski saw that he had first to investigate presentations in general, and thus to dispel the ambiguities that the notion of ‘the presented’ carried with itself (Twardowski 1926, 10). Work on concepts is to be found in the manuscript Logik 1894/5, where concepts are defined as presentations with well-defined content, in Images and Concepts (1898) and in The Essence of Concepts (1924), where presentations with well-defined content (i.e. fixed by a definition) are called ‘logical concepts.’ An important role in Twardowski's theory of concepts and definitions is played by the notion of presented judgment, i.e. judgments not really passed, but merely presented (in the modified sense). Twardowski's investigation of the notion of (logical) concept is important to understand his attitude towards objectivity and unicity of meaning, i.e. his relationship to psychologism on the one hand, and the role of the theorizations in Content and Object of two notions: that of the object of a general presentation (which disappears for instance in Logik 1894/5) and that of indirect presentations on the other hand.