Can the study of ideas in international politics be made scientifically respectable? The question is central to the Third Debate, yet the dominant voices in the ‘debate’ seem oddly to agree. ‘Positivists’ are sceptical because ideas seem ephemeral, difficult to measure, and generally resistant to hard science. As a result, positivist theories of international politics tend to favour seemingly more objective material factors like military and economic capabilities, and only bring in ideas as a last resort. In this way positivist epistemology shapes international ontology. Against this tendency, ‘post-positivists’ argue that it is simply a mistake to think that ideas can or should be studied in the same way we study physical objects. Ontology should determine epistemology, not vice versa. However, in developing this important insight many post-positivists have gone further, to efface any connection between their subsequent work and science—Understanding versus Explanation. The ironic result is to echo the positivist feeling that the study of ideas cannot be made scientifically respectable.