Philosophers of Science have long paid lip service to the desirability of distinguishing the questions that arise about the propositions (statements, hypotheses) of a science from those that arise about its concepts (terms, ideas); but hitherto there has been a curious hesitation on their part to explore the consequences of this distinction as far as they will take us. This hesitation is understandable in those writers whose primary commitment is to the methods of mathematicallogic, with its formal analysis of prepositional systems and relations. But it extends also to those, who have no such commitment: e.g. the pragmatists. (Recall William James's confused question, “What makes an idea true?” - as though concepts could be true-or-false, in the way propositions are!)

The account of scientific change and rationality outlined in this paper,. and set out at length in my recent book on Human Understanding, is designed to show how a full understanding of those consequences can help us to move beyond current quandaries in the subject to a more constructive set of philosophical, historical and sociological questions.