Plato’s Theory of Forms refers to an ideal version (which he called a Form or an Idea) of something that all objects within the physical world are merely imitations of. The world a person knows through his or her senses is only an imitation of the pure, eternal, and unchanging world of the Forms.
An example helps to illustrate this idea. Consider a square and that with the idea of a perfect square in mind a person might try to sketch it on paper. However, the sketch would almost certainly turn out to be imperfect in some way (perhaps the sides are not even or they are a bit skew) and it will thus be an inferior copy of the perfect square. The perfect square is the Form of a Square. The Form Square refers to a perfect square that is perfect in all possible ways. Similarly, like the square, everything else in the world (circles, dogs, humans, tables, trees, and concepts of justice and beauty) are imperfect representations of their perfect Form. For instance. it is only the Form of Justice that is perfect justice; it is only the Form of Beauty that is perfect beauty; it is only the Form of Square that is a perfect square.
Plato went further than to settle with the idea of a perfect Form. He believed that Forms exist independently of the human mind; they are not just imaginative ideas but are imperceptible yet permanent, reliable, and unchangeable realities that structure the world and our knowledge of it. There are two different and very real worlds: on the one hand, there is the world of material things and then, on the other, there is the transcendent world of Ideas or Forms. For Plato, only those people who are able to recognize the Forms can comprehend the true reality behind the world of everyday experience and claim true and reliable knowledge of the world. Plato uses the example of a ruler who, in order to rule successfully, must recognize the Form of Goodness.
Plato’s most famous analogy of the Forms is his allegory of the cave Republic. Plato presents the scenario of several captives being held chained to a wall in a cave. They have their backs to the cave’s mouth and just behind them is a fire. Every time an object meets the fire it casts shadows onto the cave wall in front of them. The captives name these shadows that they perceive to be reality. When some of the captives exit the cave they perceive reality anew with the help of the sun and then realize reality as they first perceived it from the shadows was not really reality at all. One of the lessons of the analogy is that human beings are bound by a limited perception of reality from which they cannot break free. If people could break free they would experience another realm of reality that would be incomprehensible and beyond their understanding. This realm is what Plato believes is the pure Form of reality that human beings perceive but as an imperfect copy.