I remember reading a whole essay by Jacques Derrida about this exact question, a...

I remember reading a whole essay by Jacques Derrida about this exact question, and how it remains open:

http://www.occt.ox.ac.uk/sites/default/files/derrida_platos_...

Plato is a great favorite of these French post-structuralist thinkers for exactly how he always leaves room for more questions, never giving you a final answer.