It is said that all of Western philosophy is a series of footnotes to Plato. Plato's influence on western culture is undoubtedly a very strong one, and this includes a strong influence on the arts, and on theories of art.
Plato developed a two-world view of reality, the changing physical world and the eternal and changeless world of Forms, which is absolute, independent, rational and transcendent. The world of Forms never changes and yet causes the essential nature of things we perceive in the physical world.
Beauty, Justice, and The Circle are all examples of what Plato called Forms or Ideas. Many particular things can have the form of a circle, or of justice, or beauty. For Plato, these Forms are perfect Ideals, but they are also more real than physical objects. He called them the ‘Really Real’. The world of physical appearances only has reality to the extent that it succeeds in imitating the Forms.
Plato saw the changing physical world as a poor, decaying copy of a perfect, rational, eternal, and changeless original. The beauty of a flower, or a sunset, a piece of music or a love affair, is an imperfect copy of the form Beauty itself. In this world of changing appearances, while you might catch a glimpse of that ravishing perfection, it will always fade. It’s just a pointer to the perfect beauty of the eternal.
Art is imitation. But what does it imitate? Art imitates the objects and events of ordinary life. In other words, a work of art is a copy of a copy of a Form. It is therefore three steps removed from reality. A painting of a bed, for example, is the imitation of a bed in the physical world that itself is an imperfect (though less so) imitation of the Form or Idea Bed, which is all that really exists.
Therefore, the works of art are even more of an illusion than are ordinary experience. They are at best entertainment and at worst a dangerous delusion. Plato was also concerned with the cognitive aspect of art, feeling that it has the effect of distorting knowledge since it is several steps removed from truth. He proposed sending the poets and playwrights out of his ideal Republic, or at least censoring what they wrote; and he wanted music and painting severely censored. The arts, he thought, are powerful shapers of character. Thus, to train and protect ideal citizens for an ideal society, the arts must be strictly controlled.