It is widely agreed that social criticism requires knowledge, preferrably from many disciplines of social research. Social ontology studies the most general questions of the nature of social reality. Different scientific approaches make different ontological commitments, and participation in everyday life may make some other ontological commitments appropriate. At least since Habermas (1968), the critical knowledge interest has been distinguished as possibly requiring its own kind of theory (cf. also Horkheimer's distinction between traditional and critical theory). These plural approaches raise the philosophical question of uniting the perspectives, but this lecture series focuses on the critical perspective, asking what kind of social ontology does critical theory require.