“There's one thing certain,” said a historian of my acquaintance when he heard the title of this paper, “that's a problem which would never perturb a working-historian.” He was wrong: a working-historian first drew it to my attention; and in one form or another it raises its head whenever historians discuss the nature of their own inquiries. Yet in a way he was right. His mind had turned to the controversies of epistemologists, controversies about “the possibility of knowledge”; historians, he rightly felt, do not trouble their-heads about such matters.