Nicolai Hartmann contributed significantly to the revitalization of the discipline of ontology in the early twentieth century. Developing a systematic, post-Kantian critical ontology ‘this side’ of idealism and realism, he subverted the widespread impression that philosophy must either exhaust itself in foundationalist epistemology or engage in system-building metaphysical excess. This essay provides an introduction to Hartmann’s approach in light of the recent translation of his early essay ‘How is Critical Ontology Possible?’ ( 1923 ) In it Hartmann criticizes both the (...) pretensions of epistemology as well as the principal errors of classical ontology, and he proposes a series of correctives that lead to his development of a highly original and elaborate stratified categorial ontology. This introduction explains the most important errors of the ‘old’ ontology, his correctives to them, and further fleshes out these correctives with reference to his mature ontological work. (shrink)