but 是一个含有否定意义的关系代词，即：它本身是关系代词，但带有not之意。

先行词指物时：but = that/which...not...

先行词指人时：but = that/who...not...

主句部分必须是否定形式或带有否定意义的词（no, little, few, hardly, barely, seldom, nobody, nothing...）。

网友句子中的but，先行词是movement。又如：

He hardly has problems but can be solved. 他简直就没有解决不了的问题。(but can be solved =which/that can't be solved)

现在，举几个先行词是人的例句：

No one is in the world but knows the Great Wall. 世上无人不知长城。
but 在从句中作主语。but knows 相当于 who doesn't know 。这样，整个句子就是双重否定，语气强于一般肯定句，有强烈的感染力。又如：
There is no one of them but is ready to help the people in trouble. 他们个个都乐意帮助有困难的人。(but is ready 相当于 who is not ready)

There is no mother but loves her own children．没有不爱自己孩子的母亲。(but loves = who/that does not love)

I find few but admire him for his talents. 我发现很少有人不钦佩他的才华。(but admire =that/who don't admire)