Begin with an elementary puzzlement. In any discussion between sceptics and believers it is presupposed that, even for us to disagree, it is necessary to understand each other. Yet here at the outset the central problem arises. For usually (and the impulse to write "always' is strong) two people could not be said to share a concept or to possess the same concept unless they agreed in at least some central applications of it. Two men may share a concept and yet disagree in some of the judgements they make in which they assert that objects fall under it. But two men who disagreed in every judgement which employed the concept — of them what could one say that they shared? For to possess a concept is to be able to use it correctly — although it does not preclude mishandling it sometimes. It follows that unless I can be said to share your judgements at least to some degree I cannot be said to share your concepts.