张三丰向无忌演示太极拳，然后问他记住多少，无忌说都忘了（小说里没有这句话，电视剧里加的）。这里的忘，是得意忘言得鱼忘筌的忘，学精神而不学形式。费曼老师讲物理，也常喜欢说忘，忘了怎么办呢？自己找回来：

you can re-create thethings that you’ve forgotten perpetually-- if you don't forget too much, and if you know enough. In other words, there comesa time -- which you haven't quite got to, yet -- where you’ll know so manythings that as you forget them,you can reconstruct themself from the pieces that you can still remember. It is therefore of first-rateimportance that you know how to "triangulate" -- that is, toknow how to figure something out from what you already know. It is absolutely necessary.

当然，费老也说了，别忘太多（don't forget too much），要能从记得的碎片还原（或重构）忘记的东西（you canreconstruct them from the pieces that you can still remember）——这儿有一个“度”的问题。Triangulate是费老对重构的形象化说法（如图），通过逻辑寻找失去或遗忘的关系——重要的是，有时还能发现新的东西。

左图是原先完整的知识，右图虚线部分是遗忘的，通过三角形重构关联。

发现原来没有的关系

那么，需要多少记忆碎片才好还原呢？显然没有好的答案。费老总是尽可能少地用别人做的东西来做自己的事情。他的口述历史说

I try always to work with the least knowledge possible, with little knowledge of what other people are doing, because I feel more happy being more individual, if I am not following the line and getting confused by what they say.

费老把自然的最小作用量原理用于自然律的创造，这是很有趣的，其实也是很“自然”的——欧几里得几何不就是那么建造起来的吗。