The view that art is mimetic was a concept long established among the Greeks. But Plato was first to compile the idea into a concise theory - the central point of it being that art being mimetic, and the art d' objects being imitation of form that was twice removed from the ideal form, the artistic practices or art forms are harmful for society. Plato would therefore not want to include poets, dramatists, painters etc. in his ideal Republic state.
By using the word 'mimesis' in his 'Poetics' Aristotle accepts that art is imitation, but that is exactly how far his view on mimesis agrees with Plato's views on mimesis. According to Aristotle, in imitating reality, art learns about the real nature and form of Nature and thus art leads to knowledge and is therefore positive for society. According to Aristotle, knowledge of reality and Nature, enables art to express what should be the ideal form and which may not really always be in Nature but which Nature tries to attain. Thus mimesis enables even Nature to be transcended through artistic endevour. The knowledge that art is imitation of reality leads to understanding of the true form in Nature and gives rise to feelings of pleasure and another mode of cathersis.