Plato wasn't the first or the last person to think that art is imitates reality, therefore; he thinks that art is imitation and corrupt the mind.
So art is imitation. But what does it imitate? Here is where Plato's two theories come in. In the Republic, Plato says that 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 even more of an illusion than is ordinary experience. On this theory, works of art are at best entertainment and at worst dangerous delusions
Throughout the Republic Plato condemns art in all forms including literature or poetry. It appears as though his reasoning is that imitation of reality is not in itself bad, but imitation without understanding and reason is. Plato felt that poetry, like all forms of art, appeals to the inferior part of the soul, the irrational, emotional cowardly part. The reader of poetry is seduced into feeling undesirable emotions. To Plato, an appreciation of poetry is incompatible with an appreciation of reason, justice, and the search for Truth. To him drama is the most dangerous form of literature because the author is imitating things that he is not.
These are the reasons which made Plato convinced with the truth that "art corrupt the mind".

