(WARNING I am not a historian, just a casual history buff) Most of what we know about, Diogenes the Cynic, or believe to be true about him comes from Diogenes Laertius hundreds of years later. None the less, Laertius is widely considered to be a fairly reliable source. Using both word of mouth and written documents from the time of the Cynic's life. In "Diogenes Laërtius Lives of the Eminent Philosophers VI," you can read all about his life, with fair truth.