Within the community of academic students of international politics today there is a deep epistemological rift over the extent to and ways in which we can know our subject. Speaking very broadly, on one side stand what have become known as 'positivists', who think we can get closer to the truth about international politics, but only if we follow the methods which have proven so successful in the natural sciences. And on the other side stand ‘post-positivists’, who think we do not have privileged access to the truth about international politics, and least of all through the methods of natural science. Although it can seem far removed, this epistemological disagreement actually matters quite a lot to our collective efforts to make sense of the real world, since we cannot avoid taking some position on it, and those positions affect the questions we ask, the methods we use to answer those questions, and ultimately the kinds of knowledge that we produce. Perhaps for this reason what may seem more a dispute for philosophers than political scientists has become one of the ‘Great Debates’ (the Third) in the discipline of International Relations (IR), and, indeed, it arguably underlay the ‘Second’ Debate between behaviouralists and traditionalists in the 1960s as well.