Full Idea
Of all actions, the ones least understood are those undertaken for a purpose, no doubt because they have always passed for the most intelligible and are to our way of thinking the most commonplace.
Gist of Idea
Actions done for a purpose are least understood, because we complacently think it's obvious
Source
Friedrich Nietzsche (Dawn (Daybreak) [1881], 127)
Book Reference
Nietzsche,Friedrich: 'Dawn (Daybreak) (v 5)', ed/tr. Smith, Brittain [Stanford 2011], p.94
A Reaction
You feel that Nietzsche is right about our stupendous lack of of self-knowledge, but then a bit of a panic ensues, because it is not clear what you are supposed to do about anything, particularly if we don't know why anyone else does anything.