Damn...I just finish an epic to Hellenic Riot on all the authors he didn't know who I listed--I'm sure he'll be THRILLED, haha--, and now I need to write about Plato...
OK, here goes.
Plato's main contributions to philosophy could--and have--fill not just a book but entire volumes, and as I've both jsut written an novel already to Riot as I said and actually have a math test that could make or break my grade that I really should be studying for...
I'll be happy to divert my attention from that math to this for a while, but not for a tome's worth, I hope. ;)
Well, Plato's dialogues serve two functions that are invaluable without even really considering the actual content or the texts of Plato himself--they recount, at least the Early Socratic Dialogues, the teachings of Scorates, and so it's through Plato's dialogues' keeping his teacher's word's alive and, many theorists agree, retain the teachings and essence of some of Socrates' ideas that we get our picture today of Socrates and where the Golden Age of Greek Philosophy, encompassing the Big Three of Socrates, Plato, and Aristotle, all began. Without Plato's dialogues, there's a good chance we lose Socrates from history altogether, and THAT has an enormous domino effect, as without Socrates you don't get Plato, without Plato's often pie-in-the-sky ideas on metaphysics we don't get Aristotle's response advocating a more down-to-Earth approach, and so without the best works of Plato AND Aristotle, the foundations upon which Western Philosophy and thought as we know it vanish or, at the very least, crumble severely.
In addition, and on a related note, while it's Socrates who makes philosophy "cool" and gives it the staying power it's--mostly--had since 500 B.C. to the present, it's Plato who gives written philosophy it's staying power (and the subsequently Aristotle who gives criticism it's staying power as well as solidifying philosophy's point by disagreeing with Plato on those matters he found either erroneous or too fantastic to make work practically, which is what Aristotle's philosophy is all about.)
Socrates gives us philsophic debate, Plato the philosophic text, and Aristotle philosophic criticism--lose Plato and you not only lose Aristotle's contribution (as what is he going to criticize if Plato's not there, he criticized and evaluated other, earlier philosohpers, but by far the philosopher who gave him his best source of inspiration to go up against was his old teacher, Plato) but Socrates as well, as, again, it's through Plato we learn of Socrates and through Aristotle we see both through a more objective lens.
So even if you dislike or disagree with every other idea to be presented here--and those I'm sure to omit, intentionally or not--the fact remains that Plato's invaluable as a torch-bearer and early developer and worker in philosophy if nothing else.
OTHER important things for and from Plato...
-First, to just magnify the Socrates-Plato-Aristotle connection and it's importance a bit, remember that Aristotle first made his mark and still is widely known for going up against Plato, most of his best works feature some form of him doing so...well, by extension, without Plato, and then without Aristotle's criticism, everyone who made their mark going up against HIM philosophically either disappears altogether or quite possibly fades in importance from the philosophic landscape, and we at least really call into question what becomes of philosophy in general...and the same goes for Plato.
-The Theory of the Forms. Everyone saw this coming, and it remains one of the greatest questions in philosophy...and I really don't know how to go into detail about it without that becoming a speech, and so I'll hold off and see if anyone WANTS me to really elaborate there, or if it's agree the Forms/Cave Analogy form an important part of not only philosophy as a whole, but crosses over into art and, before Descartes had the chance to utter that he was because the had some sort of thought about something somehow for some reason, it's this idea of seperating the physical from the metaphysical, or at least the Abstract ideas from the Concrete manifestations of those ideas, and finding the value, or lack thereof, in each that gives meainging to that immortal phrase..."Is this the real life? Is this jsut fantasy?" Is this the real existence that matters, here and now, or just fantasy, the shadows of true existence on the cave walls of our limited perception--YOU be the jduge! ;) (And for you math people out there, this is actually an EXTREMELY important notion, since Plato's Theory of Forms states that the Abstract ideas are more real, more important than any Concrete manifestations...as a result, the abstract idea of "3" or "3+3=6" is logically solid, and we don't have to have empirical qualms about dealing with that which isn't empirically concrete...and for those that are religious...)
-"The Phaedo." After "The Republic," this is quite possibly Plato's most enduring works, and one of his most important, as his character of Socrates is on his deathbed and, for his final moments of happiness on Earth before drinking the hemlock and dying...decides to talk some metaphysics and philosophy! ;) But this is more than just Plato giving his hero and former friend a great literary sendoff and tribute, but a deep and actualy very difficult account of the afterlife, souls, the nature of life and death, and what it even MEANS to be alive or dead, existent or non-existent, and if such states can last forever, and if they can't...well, what then? What makes this even more important is that Plato argues FOR the existence of some kind of soul, and does so with a variety of arguments, some of which are relatively weak or flimsy but, as this is a dialogue, serve to set up the truer points Plato ahs to offer, which are far stronger arguments, some of whihc are still debated by this day. This is, without a dobut, the strongest and most widely-acclaimed and recognized argument for a soul and a life after death in history that's DIVORCED from religion, and that, ironically, may be what gives it such stayinbg power with us--while we debate on religious grounds whether or not a God exists, or a Heaven or Hell, and so on, "The Phaedo" offers a purely logic-based argument for the soul, no God attached, so to speak, so believers and non-believers alike for centuries have looked over Plato's dialouge and tried to use it or refute it.
-The Socratic Dialogue Form. Named after Socrates, of course, the old man may have started the verbal version of this, but it's Plato who put it into writing and solidified the form. Why is this important? In the narrower gaze, some plays, such as "Man and Superman" by George Bernard Shaw, which deal heavily in philosohic themes, draw from that form, and David Hume centuries later would use it to write a dialogue on what would today be termed Creationism vs. Natural Selection, and it's in THIS dialogue where Hume comes very close to saying the things Darwin would a century later...so Plato's Dialogue Form itself has had a good deal of impact just on how philosophic and artistic arguments are presented.
-"The Republic." Just...just "The Republic," if you want me to go into the specific whats and whos and wheres and whys, I will, but that's a good-sized post in itself, likely, and I don't know if folks want to read that, especially when I've already mentioned time and again at least a few of the things that I feel are not only vital, but ingenious ideas that, properly implemented, could very well lead to a better society and, on the flip side, implemented incorrectly have led to some nasty chapters in human history. "The Republic" is probably the most important work of Western Philosophy, and so that adds a great deeal as to why I'd consider Plato a great philosopher...with the arguments he presents, and a book that's as hailed as "The Republic," that'll tend to catapult you higher in my evaluations... ;)
-Inspiration for the Elitists. This may seem a bad reason to view Plato in a positive light or as a good thinker, but I don't believe so; something Plato and Aristotle actually agreed on was the idea of Elitism, and in Ancient Greece were able to present their ideas quite openly and unabashedly. I've said before I like Nietzsche and Mill a great deal...well, the Ubermensch of Nietzsche can find its earliest roots in Plato's Elitist Philosopher King--straight from "The Republic!"--and, on the flip side of that ethical/political debate, Mill's main arguments and the bulk of his work deals, not directly with Plato, but with a refutation of that idea of an elite few being "better" than a functionalized, stagnant many.
And......that's all I'll put for now, I'll see how that goes before adding more ramblings...oh, nuts, now I have to return to math study! (If we had PLATO'S system of education I wouldn't HAVE to study for math, I'd be taking 6 English classes and any person next to me in class hating English who'll destroy the curve on the test could just take 6 Math classes..."likes with likes!" Teaching according to talent and desire!) :)