Almost always paired with 知其然 - the four-character set phrase 知其然，不知其所以然 ('know that it is, not why it is') is one of the most common ways the word appears in modern writing and speech.
history
The phrase 知其然，不知其所以然 echoes the 朱熹 (Zhū Xī, Song dynasty) gloss style on classical texts, where 然 is 'so / thus' and 所以 is 'that by which' - together literally 'that-by-which-it-is-so'.
