美国前国防部长Donald Rumsfeld有段名言：

There are known knowns; there are things we know we know.We also know there are known unknowns; that is to say we know there are some things we do not know.But there are also unknown unknowns – the ones we don't know we don't know. 

这是２００２年二月就伊拉克核武器问题讲的，其中要害当然是“unknown unknown”了。查了一下，这一提法的最先使用是在１９８４，一名美国空军准将在一篇关于军事训练的论文里用它来强调现代战争中不可预见因素。

子曾经曰过：知之为知之，不知为不知。至于这“unknown unknown”嘛，老夫子显然没想到。所以说，美国这龙头老大的地位还真不是白给的，它在战略上是站得很高，看得很远的，是大智慧，国人切不可妄自尊大。