“It is not easy to tell what exactly Hartmann understood by his ‘ontology,’ which he wanted to oppose to the old Pre-Kantian form of ontology. He certainly did not identify it with metaphysics. In this respect Hartmann’s enterprise differed fundamentally from the many more or less fashionable attempts to resurrect metaphysics, attempts which have rarely led to more than tentative and precarious results. Superficially Hartmann’s ‘ontology’ may seem to be nothing but what it meant to Aristotle: the science of being qua being in its most general characteristics. In order to determine its actual content, however, it will be best to look first at the type of topics and problems which Hartmann took up under the time-honored name. They comprise not only being qua being, i.e., the most general concept of what is (das Seiende), but existence (Dasein) and essence (Sosein), which he calls Seinsmomente, and the types of being designated by the adjectives ‘real’ and ‘ideal,’ named Seinsweisen, all of which are discussed in the first volume of the ontological tetralogy. The second volume deals with the modes of being (Seinsmodi) such as possibility and actuality, necessity and contingency, impossibility and unreality — particularly impressive and perhaps the most original part of the set. The next major theme is the categories, first the general ones applying to all the strata (Schichten) of the real world and explored in the third volume (Der Aufbau der realen Welt), then the special categories pertaining only to limited areas, such as nature, which Hartmann takes op in the final work.
Finally, there are the categories peculiar to the realm of cultural entities (geistiges Sein) which he discussed in a work whose publication actually preceded the ontological tetralogy.
The mere mention of these topics will make it clear that such an ontology differs considerably from what had passed as ontology before Hartmann. It covers more and less. It adds the spheres of being which have been opened up by the sciences and the new cultural studies as well as by the theory of values. But it omits the traditional metaphysical problems, i.e., the ultimate questions dealing with God and immortality, which were the prize pieces of speculative metaphysics. The fact that Hartmann abandoned this earlier metaphysics did not mean that he denied its problems. Their insolubility even provides the very background for his new ontology. Hence we have no right to simply ignore them.
Ontology thus conceived constitutes really a segment of a metaphysics which is no longer simply a field for speculative treatment by a priori methods. To Hartmann metaphysical problems are those which form the horizon of scientific knowledge, and which are inescapable because of their connection with what we can know scientifically, yet which cannot be solved by the methods of science alone. Some of these problems he considered to be impenetrable and ‘irrational’ on principle, even though they too contain an ingredient (Einschlag) which can be explored by the rational methods of critical ontology. This ‘least metaphysical part’ of metaphysics is the proper field of the new ontology.”
From: Herbert Spiegelberg – The Phenomenological Movement. A historical introduction – Martinus Nijhoff – The Hague, 1963 (third edition). pp. 309-310.