 Brentano’s main goal was to lay the basis for a scientific psychology, which he defines as “the science of mental phenomena” (PES, 14) [“Wissenschaft von den psychischen Erscheinungen”]. In order to give flesh to this definition of the discipline, he provides a more detailed characterization of mental phenomena. He proposes six criteria to distinguish mental from physical phenomena (PES 61–77), the most important of which are: (i) mental phenomena are the exclusive object of inner perception, (ii) they always appear as a unity, and (iii) they are always intentionally directed towards an object. (The other three criteria are: psychological phenomena – and only those – are presentations or phenomena based upon presentations; they seem to have no spatial extension; and have not only intentional, but also actual existence.) I will discuss the first two criteria in the next sub-section, and the third in a separate sub-section below. 