Socrates, Plato and Aristotle
Let's just getting the ball rolling...
There are no actual writings from Socrates. Most accounts of the philosopher are taken from the writings of his student Plato. The main concept that Socrates pioneers is also a method named after him - The Socratic Method
At its base, the Socratic method is deceptively simple: question all weaknesses of an argument until a gap in reasoning is revealed. That gap is an opportunity to learn.
Now in practice, this is a much more difficult concept to implement as it takes exceptional mental awareness and a willingness to face the truth. These alone are what drive the citizens of Athens into accusing Socrates of poisoning the minds of the youth because he was revealing them as fools who believed in ideas they could not support with reason.
The oracle told Socrates that he was the wisest of all people. In typical Socratic fashion, Socrates decided to test this premise. He went to the artisans in town and started asking them questions about their craft and what they believed in, hoping to find someone without a flaw in their beliefs. Over time, he exposed each of the artisans as people who really didn't know as much as they thought they did, so Socrates proceeded to move on to the merchants in the town. Again, he met the same situation time and again - exposing each in turn. Finally, he turned to those in the government...surely these wise men who ran Athens would have at least one among them with no flaw in his beliefs? Sadly, the same results as the previous two attempts. By this point in time Socrates has turned most of the town against him for exposing visible members of the community as fools. Socrates, realizing that the oracle was right then set upon himself, coming up with the conclusion that he was the wisest man because he knew that he knew nothing whereas others were deluding themselves that they knew something but upon closer inspection did not. The climax of this tale ends with Socrates being brought up on charges and in his defense the court asked him what he thought his punishment should be. Socrates replied that he should be given room and board, at government expense, for the rest of his life (an honor usually given to Olympians/heroes). Incensed at not only being made fools of, but of this ridiculous request, the court made judgement that Socrates either be banished from Athens or made to drink hemlock (poison). Since being banished from Athens was considered a cop-out and would be considered today as being exiled to Siberia or Antarctica, Socrates chose the hemlock. In another of Plato's writings, there is a discussion about Socrates possibly going on a jailbreak; however he refuses, citing personal morals and conviction as cornerstones of humanity.
The Platonic Cave
The Platonic cave is analogy that Plato makes in 'The Republic' to demonstrate that the truth of particular things in the known world lie in their abstract form, for example that the beauty of a painting participates in the objective idea of beauty and not the opinions of a person. The analogy is as follows; imagine a group of prisoners are imprisoned within a cave and cannot even see each other. The only knowledge they gain is from objects moving across a bridge whose shadows are illuminated by a fire. Thus all that the prisoners know are the shadows of real things. Now imagine one of the prisoners, let us call him Socrates, breaks free one day. Socrates goes out into the world outside the cave and sees things as they really are. He sees not a shadow of a tree, or a person but actual trees and people. Socrates rejoices in the fact that he has seen the true forms of real things and after a time studying objects in the world returns to the cave to tell the other prisoners. Upon doing so the prisoners cannot comprehend the true reality of things, they know the shadows on the cave wall and cannot accept the reality of objects. They kill Socrates for trying to convince them otherwise. As an analogy the cave works as a metaphor for the world in which each individual human being lives. Socrates is the only person able to escape and is a metaphor for the philosophers who are able to glimpse the true reality of things. In other texts Plato describes the world of the forms or ideas, that is the world in which things exist as they actually are, as what always is but is never becoming. In other words such a reality is eternal in nature due to its abstract form. In contrast the particular world, the cave, is the world in which everything is always becoming but never is. Meaning that all objects of the world are always trying to become like the forms but due to the fact that they are not objective can never be exactly like the objective forms which they represent. A beautiful painting can never be actually beautiful, it can only ever be a close approximation to beauty, hence why not everyone finds it beautiful. Some debate is held as to whether Plato actually meant that there are two separate worlds, one of forms and one of particular things, or whether it is all part of one world in which the abstract forms exist eternally and particular thing finitely. For example formulas gained from studying geometry are abstract and always true in nature but the actual shapes in nature only participate in a likeness of such a formula.
Forms as Cause
Plato suggests that the cause of all particular things in the perceived world are the ideas or forms. This is due to the need of particular things to participate in something abstract that is in perfect being. The existence of the forms themselves is due to the fact that they are perfect and good in themselves. As the worst thing for a perfect existence is its non-existence it must exist always out of necessity. Hence there is always something rather than nothing and no thing can be caused by nothing what so ever.
Plato was influenced by two accounts of nature that are seemingly apposed. The one, Hericlitus, suggested that everything in the world is in a constant state of flux, that is always in motion and ever changing. Thus no one can stand in the same river twice. In contrast Parmenides has an account of nature suggests nothing is in movement, the world we perceive is an illusion of movement. Plato's abstract forms constitute a state where nothing is in movement, whilst at the same time the things participating in them are in flux out of necessity.
Aristotle was a pupil of Plato's academy. Upon not becoming the head teacher of the school after Plato's death he set up his own school of thought that differed in its way of thinking. Aristotle was an epistemologist believing that the truth of objects in the world was to be found in objects themselves, not in their separate abstract form as Plato has suggested. This claim can be seen as follows; Socrates never thinks without an image present to him. All truth is present within objects in the world. Thus any truth of the objects must be contained in the images Socrates has present to him. The study of metaphysics, the study of being qua being, (Qua meaning the capacity or role of an item) or first philosophy reveils truths to us present in the objects.
Aristotle's Rejection of the Platonic forms- The Third Man Argument
Aristotle formalizes in his 'Metaphysics' a rejection of Plato's theory of the forms known as the Third Man Argument, although it is found in one of Plato's own works 'Parmenides'. It is as follows: if particular things participate in the likeness of forms then they cannot be completely different, Socrates cannot be completely different from the form man else he cannot be participating in a likeness of man. If they are the same then Socrates is equivalent to the form man and nothing is added to the argument by the form. Thus Socrates must be similar to the form man, not equivalent to nor completely different to the form man. But such likeness, Man 3, is a third thing between the form and the particular, again this likeness cannot be equivalent or completely different to the form, due to the above arguments and so a fourth likeness is needed to explain the similarity, Man four. There is an infinite regress in which there is always a need for another likeness. The effectiveness of the third man argument can be disputed. If Socrates participates in the form man his similarity is not one of similarity given to Socrates but by trying to be like it. This is something Aristotle himself posits in his concept of matter and God, when all matter is trying to be like the first cause, God but can never actually be God itself. Never the less for a long time, particularly the medieval period when Aristotle was known as 'The Philosopher', this argument was seen as logical proof for the non-existence of abstract forms as apart from particular things. This is not to say that there are not forms in which all things participate.