这个 but 其实是一个表示否定意义的关系代词（= who / that…not）。如：
There is no one but knows it. 没有人不知道这件事。
=There is no one who doesn’t know it. 没有人不知道这件事。
又如：
There is no man but has his faults. 人皆有过。
There is no rule but has exceptions.
没有无例外的规定。
There is no tree but bears some fruit. 没有树不结果。
Not a man but had tears in his eyes. 没有人眼中没有眼泪。
There is no man but feels pity for starving children. 没有人不同情那些嗷嗷待哺的孩子。
Surely there isn’t a mother but faces this problem. 肯定的，没有一个母亲不面临这个问题。
