 After covering the modal categories that allow us to distinguish between ideal and real spheres in PA, and exhaustively covering fundamental categories and categorial laws in A, Hartmann covers the special categories of the two lower strata in N. Hartmann had already worked out a number of special categories belonging to the higher strata in earlier work, including the Ethics and the Problem of Spiritual Being. Philosophy of Nature examines the ontological dimensions and categories of the cosmological, physical, and biological sciences of his day. Paul Feyerabend, while otherwise critical of Hartmann, praises Hartmann for developing a coherent “philosophy of process” in this book (Feyerabend 1963, 101). In it, Hartmann declares that “mathematical physics deals with reality qua measurable, not with reality as such” (N24), and further develops his complex systems conception of living things. Hartmann’s work in these areas was influential for scientists like Konrad Lorenz (1903–1989) and Ludwig Bertalanffy (1901–1972), who explicitly refer to him. I’ll summarize the special categories of the strata here before closing with a few open questions about specific aspects of Hartmann’s ontology.