Socrates is known also by other contemporaries. Aristophanes wrote a comedy, 'The Clouds', where he lampooned Socrates. Xenophon, a contemporary (and the author the Anabasis), published severat 'books' on Socrates, including dialogues. They do differ from Plato's, though. Whether the historic Socrates was the Plato or the Aristophanes or Xenophon man - we don't know. That he existed is almost certain, and that he was an extraordinary being, is also certain - being discussed and referred to makes that certain.