I am finally given the opportunity of correcting an error I made a few years ago when I claimed that Oppenheim's conception of realism was 'totally in tune with the one H. Morgenthau proposed 40 years ago in his famous "Six Principles of Political Realism", listed in the first chapter of his Politics among Nations". Oppenheim pointed out to me in a letter: "I do not agree with Morgenthau. He considered the pursuit of the national interest a moral duty, whereas I view it as a practical necessity, hence neither morally right nor wrong, since ethics is meaningful only where there is choice". What was the nature of my mistake? Let me say right away that the target of my argument was not one or other of these authors, but political realism as such, in the context of international relations. From this perspective one can indeed say that Morgenthau and Oppenheim share the view that the defence of the national interest has and must have absolute priority over any other kind of consideration, while nevertheless admitting – as I indeed do – that whereas Morgenthau holds the defence of the national interest to be a duty, Oppenheim treats it simply (or at least alternatively) as a rational requirement of action on the part of the politician or statesman.