Before Plato, a few other philosophers had tried to explain the metaphysics of the universe and amongst them Parmenides, who thought everything is one, and Heraclitus, who thought everything is always changing, are important names. We find, as we read Plato, that it is hard to distinguish where Socrates’ metaphysics ends and where Plato’s start. Plato distinguishes the real from the concrete in the Universe and then constructs his Theory of Forms. Firstly, he distinguishes everything in the universe into either a universal or a particular. Universals are the characteristics and qualities that all particulars have in common. The particulars are things like dog, paper, horses, etc. However, Plato believes that there happens to be something somewhere that can be used as a point of reference to establish that all the different sorts of dogs in the universe are after all, dogs. The dog-ness of the dog is universal and helps us identify a dog.
In Plato’s time, people were of the view that real things are ones that can be touched and felt or more simply, perceived by the senses. The concept of concrete is a simple one. Concrete are sensible, material particulars and we know that everything that is concrete in this world, e.g. chair, table, computer, wood, etc. will all decay and meet an end. Concrete is temporal. It is perceived by senses but according to Plato our senses do not determine what is real and what is not. The opposite of concrete is abstract which includes things like freedom, justice, virtue, etc. These abstracts are eternal and have the same meaning. They are much more real than what our common sense calls real.
If we properly examine a white paper, we will find that it is not truly white but has grey or yellow spots along the area. The white paper is hence imperfect. Also, we know for sure that the white paper was not always there and will not always be there. So the real that people have perceived may not really be real. Plato found the need to determine the real; the one that does not change and decay over time and one that is truly real. We know the white paper we examined was not really white but we have an idea of white since we compared the idea with the paper to determine it is not really white. The idea of a white paper is real and eternal. It is the very same for everyone and remains so. Another example is of a straight line that we draw; it may look straight but it never is. Straight lines are always flawed but the idea of the straight line we have is real. Now that real is not in this world because everything in this world is concrete and it is decaying. Also, a very common example helpful to explain what Plato meant by real is that of language. We use different languages and yet convey the same message in all of them. The message that we convey is common in all languages and it is real.
The Allegory of the Cave is a very important analogy that Plato writes about in his book VII of The Republic. It is told through Socrates. He tells us about a cave with tied prisoners that are facing the wall and cannot move. There is a fire behind them they cannot see and behind the fire there are puppeteers that are holding puppets and a shadow is being cast on the wall. Prisoners are facing that wall and so they only see the shadows and hear the noises made by the puppeteers. They will easily consider the things on the wall as real. Their senses perceive only what is apparent and they will know nothing about the real cause of the shadow. So what they assume as real is not really real. Plato thought this was the case with people; they thought the concrete real but it is only something we can perceive and our perceptions do not determine real.
Plato believes that Forms or Ideas are real. What are forms for Plato? The real abstract universals that may not be easy to perceive but they can be conceived. They are a part of this universe but not this world. However everything concrete in this world is only a reflection or a shadow of the real abstract universals present in the alternate universal realm. Concrete is not real for Plato and real is abstract.
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 Translated by Benjamin Jowett, Plato’s Theaetetus, (Oxford, Oxford University Press: 1861), P.60
 S. Marc Cohen, The Allegory of the Cave, (http://faculty.washington.edu/smcohen/320/cave.htm: May, 2011)
 George Grote, Plato and the Other Companions of Sokrates, (London: 1865), P.442