Is it possible to deduce how much of Platos philosophy is original to himself and not due to his predecessors?

Question

Alfred North Whitehead said 'The safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato'.

I'm not interested (in this question) as to what Whitehead meant by this, but simply that he points towards Plato as the pre-eminent ancient philosopher in the European tradition.

Before Plato there appears to be several schools of philosophy primarily concentrated in the colonies of the Greeks. Whereas Plato is based in Athens, the pre-eminent polis of the time.

How much of Platos dominance is due to the glamour of Athens, and presumably (in contrast) the shabbiness of the colonies. For example, it may mean that his work was more often copied and commented on because he worked in Athens, and not simply because of his originality (if any). And how much of his work is a gathering together rather than a synthesis of diverse philosophical positions developed elsewhere.

It maybe that this question is unanswerable, but it does seem to me to be an obvious question that may have been addressed by an expert of ancient greek philosophy.

Addendum: Guthrie in his A History of Greek Philosophy – The Presocratic tradition from Parmenides to Democritus (1979) has this to say:

Few would deny originality to Plato, yet his philosophy could be plausibly represented as arising simply from reflection on the utterences of Socrates, the Pythagoreans, Heraclitus & Parmenides. What distinguishes the Hellenistic systems is a difference of motive.

Also the SEP entry on Heraclitus has:

From an early time Heraclitus was seen as the representative of universal flux in contrast to Parmenides, the representative of universal stasis. Cratylus brought Heraclitus' philosophy to Athens, where Plato heard it. Plato seems to have used Heraclitus' theory (as interpreted by Cratylus) as a model for the sensible world, as he used Parmenides’ theory for the intelligible world.

Answer 1

First: if one wishes to examine the writings of the known pre-Socratic philosophers, these have been widely published and are readily available. From these, it is difficult to find anything that Plato borrowed from anyone prior to Socrates.

Next: if one wishes to look at the contemporary documentation of Plato's place in Greek philosophy, one must turn to Aristotle; this is the only significant source we have. Naturally, this is skewed by the fact that Aristotle was a student of Plato's.

Then: if one wishes to take a broader historical view, one can turn to Diogenes Laertius, who remains the best (relatively) early historiographical source regarding Greek philosophy; however, he was writing many centuries after Plato, and although many of the sources he relied upon are now lost, we have no way of knowing how many other sources were lost before his time.

And this takes us to our final, Rumsfeldian point: there's no way that we can possibly know how many philosophers or texts are lost to history-- we necessarily know only that which has survived.

However, I would suggest that there's no particular reason to suspect that Athenian chauvanism was a major factor since many of the pre-Socratic philosophers that have come down to us are from other places (such as the Ionian school, or the Eleatic school, etc.) and these appear not to have been suppressed.

Answer 2

The pre-socratic philosophers (also known as phyσical philosophers, the materialistic thinkers, mainly appearing in Ionia) influenced Plato considerably. In essence, his whole admiration for "ideals" and "sacred numbers" and "notions" is a response to their attitude for love of nature and its phenomena.