This study discusses Brentano’s influences on Twardowski’s psychological project. The author shows that Twardowski’s psychology is not unitary. In his early writings on psychology and his Lvov lectures, Twardowski developed psychology mainly in the Brentanian fashion. To show this, the author focuses first on the question of the subject-matter of psychological inquiry and its method in Twardowski’s early writings, namely, introspection. Secondly, the study reconstructs elements of Twardowski’s mereology and traces how he used mereological devices to argue against Brentano. The author presents how Twardowski sketches a sharp distinction between the object and content of presentations and reconstructs his arguments for adopting a four-class taxonomy of mental phenomena (instead of Brentano’s three-class taxonomy). Later, around 1910 - 1913, Twardowski redefined the terminological background of psychology. Finally, the chapter explores Twardowski’s late theory of psychology. The aim is to track not only Twardowski’s continuation of but also his breaks with Brentano’s project.