The theme of the 2006 IAP Conference, held at Alexandria, from 1-3 December 2006, was 'The Unity of Science'. Let me explain how I interpret this interesting motto. For me, this does not mean that there is one regina scientiarum that rules over the other fields of science and learning; a role that was allotted to theology in olden times, and that nowadays - in a more secular vein - is claimed by physics. In my view, the notion 'unity of science' rather refers to 'communality within diversity'. Disciplines vary in content, issues and methods. But there are also quite some common objectives, interests and concerns, the most important of which may be the common goal of searching for testable truth with objective and independent evidence. The communalities render it possible, or even imperative, to communicate and to cooperate. The diversity implies complementariness and calls for interdisciplinarity in the study of today's numerous and complex phenomena in science and society. © 2008 Academia Europaea.