The story: [Republic: Book VII by Plato]
Imagine, you live in a cave. Your whole life has been here. You are chained to face a dimly light wall. Every so often a shadow passes by. This is your reality. Others who share your life speak with you and talk about these shadows. You give the shadows names like Cat, Horse, Car, Plane etc. Everything you can conceive, at this point, is in the shadows.
One day you are torn from that chair and turned around.Blinded at first you see nothing. As your eyes adjust you see that all this time there was a fire. In front of this fire passed statutes. These statues are even more perfect forms than the shadows. You now see what a cat, horse, car and plane look like. You think to yourself how foolish was it to believe that the shadows were all that there was.
Again, you are being dragged away. This time out of the cave. You are blinded even more so than from the fire. After your eyes adjust, the first thing you see are the shadows across the ground. You see forms more perfect than the statutes and are overcome with new insight. The sun shines over all of these perfect forms and you feel obligated to tell your fellow dwellers.
You return to the cave and tell the people what you have learned. You tell them that the shadows are not truth, they are imitations of the truth. To them, you look drunk. You sound like a fool. The ramblings of a mad man, who has lost his mind. Burdened with fear, they stone you to death for your heresy.
Everything that existed in the cave is what Plato is describing as the sensible realm. This is what we can see and what we believe based on senses. The shadows are our senses and merely imitations of what he calls “the form”. The imitations are often referred to as particulars. The form’s; which govern the particulars, exists within the intelligible realm.
This realm of the intelligible is the exterior of the cave; where the shadows are thought and the real manifestations are the forms of everything. This is the realm of thought and understanding, where the sun (his form of the good) brings light to the perfect, immutable, eternal and intelligible forms.
The form of the good is what Plato believes is the perfection of all forms. Everything that is manifest into a particular is governed by the good. Particulars can be governed by multiple forms, and are governed to greater and lesser degrees.
This idea of forms and particulars is Plato’s epistemological and metaphysical argument. This is how he describes reality and knowledge. He expresses his argument in this way to show that seeking knowledge is a difficult path, as it was for the escaped dweller. Also, that when explaining your new founded knowledge you can be branded a fool by others, and even killed like his mentor Socrates. (Curiously enough that is the character he chose to tell this tale.)