Possible Clues:

• Greek philosopher there's a lot about on the front page
• Dialogue writer
• "Phaedo" philosopher
• Greek Academy founder
• He wrote 'Knowledge is the food of the soul'
• Republic author
• Philosopher left a shirt in river
• Aristotle was his student
• Teacher of Aristotle, pupil of Socrates
• Figure in Raphael's 'The School of Athens'