Classical realism is enjoying a renaissance in the study of international relations. It is well known that the analytical and normative international-political thought of early 20th-century classical realists is based on assumptions about human nature. Yet current knowledge of these assumptions remains limited. This article therefore revisits and examines the nature and intellectual roots of the human nature assumptions of three truly consequential classical realists. The analysis shows — similar to the causa Hans J. Morgenthau — that the human nature (...) assumptions of George F. Kennan, Walter Lippmann and E. H. Carr are — to varying degrees — Freudian. Unless this hitherto neglected dimension is recognized, an understanding of their international-political thought and of the political philosophy of realism remains incomplete. Two conclusions are presented. First, current criticism of classical realism’s human nature assumptions is unjustified. Second, any (re-) engagement with classical realism must recognize its human nature baggage. Realist international-political theorizing is necessarily human nature-based international-political theorizing. (shrink)