"We have now to operate upon what we ordinarily operate readily and unquestioningly with. We now need the theory of our daily practice, the geography of our daily walks. When two or twenty familiar implication threads seem to pull across and against one another, it is no longer enough to be able unperplexedly to follow along each one by itself. We need to be able to state their directions, their limits and their interlockings; to think systematically about what normally we merely think competently with."