我坚信一个理论，那就是只有当你在享受乐趣时才能发挥出最佳水平。

The utility of an idea determines its place in mental life.

A theory should not attempt to explain all the facts, because some of the facts are wrong.

译文：连续地打仗，百姓就会感到劳苦；长期打仗，士兵就会感到疲惫。