It is true that we cannot be 100% sure of what Socrates said himself as Plato wrote The Apology. However, as this is an account of a well known event, we can be sure that it is accurate (many other of Socrates' friends were present and Plato is less likely to have written something different when there were other people who witnessed the speech). Anyway, the Socratic Problem usually only arises in the dialogues that Plato wrote, not accounts. Plato did use Socrates as a "character" for delivering his own philosophical treatises as well as some of Socrates'. Many historians believe "The Apology" to be Socrates own.
This is the foundation to most, if not all, modern thinking.
Aristotle claimed that the philosophy of Plato closely followed the teachings of the Pythagoreans,  and Cicero repeats this claim: "They say Plato learned all things Pythagorean" ( Platonem ferunt didicisse Pythagorea omnia ). 
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