Existentialia and categories are the two basic possibilities for characters of Being. The entities which correspond to them require different kinds of primary interrogation respectively: any entity is either a "who" (existence) or a "what" (presence-at-hand in the broadest sense). The connection between these two modes of the characters of Being cannot be handled until the horizon for the question of Being has been clarified.
In our introduction we have already intimated that in the existential analytic of Dasein we also make headway with a task which is hardly less pressing than that of the question of Being itself-the task of laying bare that a priori basis which must be visible before the question of 'what man is' can be discussed philosophically. The existential analytic of Dasein comes before any psychology or anthropology, and certainly before any biology. While these too are ways in which Dasein can be investigated, we can define the theme of our analytic with greater precision if we distinguish it from these. And at the same time the necessity of that analytic can thus be proved more incisively.
¶ 10. How the Analytic of Dasein is to be Distinguished from Anthropology, Psychology, and Biology
After a theme for investigation has been initially outlined in positive terms, it is always important to show what is to be ruled out, although it can easily become fruitless to discuss what is not going to happen. We must show that those investigations and formulations of the question which have been aimed at Dasein heretofore, have missed the real philosophical problem (notwithstanding their objective fertility), and that as long as they persist in missing it, they have no right to claim that they can accomplish that for which they are basically striving. In distinguishing the existential analytic from anthropology, psychology, and biology, we shall confine ourselves to what is in principle the ontological question. Our distinctions will necessarily be inadequate from the standpoint of 'scientific theory' simply because the scientific structure of the above-mentioned disciplines (not, indeed, the 'scientific attitude' of those who work to advance them) is today thoroughly questionable and needs to be attacked in new ways which must have their source in ontological problematics.
Historiologically, the aim of the existential analytic can be made plainer by considering Descartes, who is credited with providing the point  of departure for modern philosophical inquiry by his discovery of the "cogito sum". He investigates the "cogitare" of the "ego", at least within certain limits. On the other hand, he leaves the "sum" completely undiscussed, even though it is regarded as no less primordial than the cogito. Our analytic raises the ontological question of the Being of the "sum".