What unites philosophers of science – if anything does – is an insatiable interest in ideas and in their history. I mean, of course, abstract ideas : those things in whose existence Berkeley did not believe, although, being a great philosopher of science, he was as fascinated by them as any of us. Abstract ideas, that is, theories, hypotheses, conjectures, and other ‘notions’, in every state of their development, from groping and confused ideas to clearly and sharply formulated ones, are the things on which we feed, and on which we thrive.