 The inter-war period was the time of his most extensive output. In his 1921 Grundzüge einer Metaphysik der Erkenntnis (Fundamentals of a Metaphysics of Cognition), Hartmann declares his independence from the Marburg school (while occupying the Chair of one of its founders) by challenging one of its central tenets: cognition does not create its objects, as Neo-Kantianism holds, but grasps something that exists independently of it. Put briefly, the Metaphysics of Cognition discusses the interaction between epistemology and ontology and the unavoidable dependence of any epistemology on ontological assumptions. He published his truly magisterial, innovative and comprehensive work on Ethics in 1926, which includes some extensive remarks on the ontology of values in some core chapters (Hartmann 2002–2004). The first volume of the commissioned Die Philosophie des deutschen Idealismus (The Philosophy of German Idealism, on Kant, Fichte, and the Romantics) appeared in 1923. Two very important essays on ontology also appeared in the mid-twenties, “How is Critical Ontology Possible?” (2012 [1924]), and “Kategoriale Gesetze” (“Categorial Laws,” 1926), which are the major precursors of his fully developed ontological views. If we set aside his continued strong output of essay-length work, then between 1926 and 1935 his major publications include Volume 2 of his Die Philosophie des deutschen Idealismus (Philosophy of German Idealism) in 1929, Zum Problem der Realitätsgegebenheit (On the Problem of the Givenness of Reality) in 1931 (reprinted in Hartung and Wunsch 2014, 177–264), as well as Das Problem des geistigen Seins: Untersuchungen zur Grundlegung der Geschichtsphilosophie und der Geisteswissenschaften (The Problem of Spiritual Being: Investigations into the Foundations of the Philosophy of History and the Human Sciences) in 1933. On the Problem of the Givenness of Reality is significant not only because it is revised and incorporated into Part III of Ontology: Laying the Foundations, but because it was originally presented to a meeting of the Kant-Gesellschaft in Halle dedicated to the “turn to ontology and realism in contemporary philosophy” (Hartung and Wunsch 2014, 181). Subsequent to Ontology: Laying the Foundations in 1935, he steadily churned out the remaining volumes of his ontological work, publishing Possibility and Actuality in 1938 (Hartmann 2013), Aufbau in 1940, and he completed the fourth volume Philosophy of Nature in 1943, although it was not published until 1950, after all of the other volumes could be republished after WWII. The Aesthetics, under revision at the time of his death, was posthumously published in 1953 (Hartmann 2014).