The epistemological principles underlying randomized, controlled trials and evidence-based medicine generally have not received the attention they require. Broadly speaking, they are the application of work done over several centuries in philosophy and scientific method. The epistemological base is sound, but it also implies internal limitations, having to do with decreasing generality, which particularly affect application to psychological problems. The principles also have nothing to say about values. The question of the 'objective validity' of scientific method is briefly discussed.