Alfred North Whitehead is often (mis-) quoted as saying that all philosophy is a series of footnotes to Plato. I was asked the other day whether this is a fair assessment, or an exaggeration. Well, the first thing to note is what ANW actually said (emphases mine):
The safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato. I do not mean the systematic scheme of thought which scholars have doubtfully extracted from his writings. I allude to the wealth of general ideas scattered through them.
Answer #1: A bit of an exaggeration, but ANW is essentially right: Plato so totally set the agenda for asking BIG questions about "how things, in the most general sense, hang together, in the most general sense" (Wilfrid Sellars) that at least in the West even mutually distant traditions (heirs of Martin Heidegger vs heirs of Bertrand Russell, say) deal centrally with problems he was the first to set out - in epistemology, metaphysics, logic, ethics and politics, and more. Plato's work is, as he says in the same passage, "an inexhaustible mine of suggestion."
It's an interesting subsidiary question whether this is due solely to his profound originality,* or (also) to the fact that his work survives entire while the work of so many other brilliant Greek thinkers (Protagoras, Anaximander, Heraclitus, Posidonius, Empedocles ...) is blowing in the wind.
* Also interesting: why does Plato seem so inexhaustible? I think partly because he has what Keats identified in Shakespeare: "negative capability." Like Hamlet, much of a typical Platonic dialogue leaves us unsure what to think about what Plato thinks. And this is not because, qua Hegel and too many other philosophers, we can't decide what he's on about. It's because there's a diamond-like multifacetedness (if that's a word, and it now is). You can say "Plato is here defending [----]ism" and make sense of it, or say "No, on the contrary, he's defending [----]ism" and make a (different) sense of it. He's frank and straightforward yet Protean and slippery. He keeps eluding us. That "philosophy never makes progress" is a favorite canard of people - leading scientists, often - whose ignorance of actual philosophy is near-absolute; that we keep going back to Plato (armed with our very new, very un-Greek philosophical ideas), and find that he still has stuff to say to us, shows how brilliant he was at identifying the tough questions.
Answer #2: A pretty wild exaggeration, for many reasons. Metaphysics started with the pre-Socratics, not with Plato more than 100 years later. Ethics started with Socrates, not with Plato, and clearly one key strand of our tradition is footnotes to Socrates, even if the guy on the bike with the courier-bag was Plato. Aristotle was Plato's pupil, but he was a deeply non-Platonic thinker who was arguably much more influential in the West for many centuries than Plato himself. Other large traditions even in the Greek world are to some degree independent of Plato - e.g. Stoicism and Epicureanism; they may owe something to Plato but are no mere footnotes. Christianity wrestled with Plato but introduced fundamentally new ways of thinking and a fundamentally new agenda. Most modern philosophy of science and mind comes out of issues that didn't even exist for the Greeks - or perhaps existed in embryonic form, but more often in Aristotle than in Plato. All modern logic emerges - and most philosophy of language emerges with it - out of discoveries by Frege and others that are only 150 years old. (We can perhaps forgive ANW for not making that point, since he and Frege were near-contemporaries.) And last but very definitely not least: many important genuine advances in our thinking that are due to philosophy (see note re smugly ignorant scientists, above) come out of the Eighteenth-century Enlightenment and are at best very vaguely presaged in his writing. (Key example: "Enslaving your defeated enemies is wrong.")
Finally, note ANW's "European" qualification. Much of what Western philosophers put at the center of the menu may come from Plato, but (at long last) other traditions are being taken seriously even in the West - e.g. Indian and Chinese and Persian and Arabic and African traditions long-dismissed by many westerners as "not really philosophy." (If Lao Tzu and Confucius are "not really philosophers," then neither are Seneca or Epictetus; this last move would no doubt be wholly acceptable to some of our more enthusiastically narrow-minded tenure-monkeys.) Some of these traditions have some concerns similar to some of Plato's - but some of them didn't know Plato existed, and got along doing philosophy just fine.