An advance in knowledge is usually thought of as the discovery of new facts. Yet our knowledge may also be said to grow when we develop fresh ways of formulating old facts. Copernicus pointed to no new stars but gave us another conceptual system for looking at the old ones. If we consult a philosopher, our concern is presumably more with a re-appropriation of what we already know than with any addition to our store of facts. In philosophy, a new period is always an "age of criticism", when thinkers are called "to undertake anew the most difficult of all tasks, namely that of self-knowledge".¹ The questions they have been asking are themselves radically questioned. Further variations in the attack and rejoinder of accepted controversy no longer seem productive, and it is asked whether the terms of the problem adequately express the genuine intent of our questioning.