Socrates

Socrates wrote nothing down. Everything quoted from him arrives through Plato's dialogues, Xenophon's recollections, or later compilers -- so a 'Socrates quote' is always at least second-hand. 'The unexamined life is not worth living' is solid (Plato's Apology, from his trial); 'The secret of change...' is a modern fabrication popularized by a fitness book. The gap between the two is the whole story of classical quotation.