Complex sentence: Since the only source concerning her is Plato, it is uncertain whether she was a real historical personage or merely a fictional creation; however, nearly all of the characters named in Plato's dialogues have been found to correspond to real people living in ancient Athens.
Split into simpler sentences: Since the only source concerning her is Plato, it is uncertain whether she was a real historical personage or merely a fictional creation. However, nearly all of the characters named in Plato's dialogues have been found to correspond to real people living in ancient Athens.