but是连词，是but that或but what的省略形式，可引导名词性从句，用于否定词或疑问词之后，表示否定意义，相当于that...not，在语义上的确有些接近whether，但but (that)所引导的从句毕竟表示否定意义，所以严格地讲，Who knows but he may go? 应译为：谁知道他不会去？又如：

Who knows but (或but that, but what) it may be so. 谁能说不会这样？《新英汉词典》

Who knows but that he may be right? 谁知道他（说得）不对?（说不定他是对的 [说得对]）《21世纪英汉汉英双向词典》

▲    费致德《现代英语惯用法词典》对这个问题有更具体的解释。摘录如下：

在动词say, think, conceive, conclude, believe, know, say, be sure等的疑问句或否定句里，but, but that, but what意味着双重否定，结果成为肯定。but这种用法现已少见。but what是but that的代用语，常用在俗语里。例如：

I could hardly believe but (but that. but what) it was all real (that is wasn't all real).我很难相信它不全是真的。

Who knows but (that) everything will come out right? 难道不是每个人都知道结局可以没事吗？