Full Idea
There is no one who knows what they ought to do, but thinks that they ought not to do it, and no one does anything other than what they think they ought to do.
Gist of Idea
People do what they think they should do, and only ever do what they think they should do
Source
report of Socrates (reports of career [c.420 BCE]) by Xenophon - Memorabilia of Socrates 4.6.6
Book Reference
Xenophon: 'Conversations of Socrates', ed/tr. Waterfield,R/Tredennick,H. [Penguin 1990], p.207
A Reaction
This is Socrates' well-known rejection of the possibility of weakness of will (akrasia - lit. 'lack of control'). Aristotle disagreed, and so does almost everyone else. Modern smokers seem to exhibit akrasia. I have some sympathy with Socrates.