I'm currently writing a paper for a class that's supposed to "weave" together the philosophical system presented in Plato's Allegory and dialogues. 

When I spoke with my professor on the assignment, he said that Plato's own philosophical opinions were unwritten and cannot necessarily be gleaned from his own writings, and thus I ought not to refer to the system found in Plato's texts as "Plato's philosophy."

Is this true? Are we wrong to attribute things like the Theory of Forms and the proto-social contract theory as described in *Meno* to Plato?