Aristotle did not invent the term "imitation". Plato was the first to use the word in relation with poetry, but Aristotle breathed into it a new definite meaning. So poetic imitation is no longer considered mimicry, but is regarded as an act of imaginative creation by which the poet, drawing his material from the phenomenal world, makes something new out of it.

In Aristotle's view, principle of imitation unites poetry with other fine arts and is the common basis of all the fine arts. It thus differentiates the fine arts from the other category of arts. While Plato equated poetry with painting, Aristotle equates it with music. It is no longer a servile depiction of the appearance of things, but it becomes a representation of the passions and emotions of men which are also imitated by music. Thus Aristotle by his theory enlarged the scope of imitation. The poet imitates not the surface of things but the reality embedded within. In the very first chapter of the Poetic, Aristotle says:
Epic poetry and Tragedy, Comedy also and Dithyrambic poetry, as also the music of the flute and the lyre in most of their forms, are in their general conception modes of imitation. They differ however, from one another in three respects – their medium, the objects and the manner or mode of imitation, being in each case distinct.
The medium of the poet and the painter are different. One imitates through form and colour, and the other through language, rhythm and harmony. The musician imitates through rhythm and harmony. Thus, poetry is more akin to music. Further, the manner of a poet may be purely narrative, as in the Epic, or depiction through action, as in drama. Even dramatic poetry is differentiated into tragedy and comedy accordingly as it imitates man as better or worse.

Aristotle says that the objects of poetic imitation are "men in action". The poet represents men as worse than they are. He can represent men better than in real life based on material supplied by history and legend rather than by any living figure. The poet selects and orders his material and recreates reality. He brings order out of Chaos. The irrational or accidental is removed and attention is focused on the lasting and the significant. Thus he gives a truth of an ideal kind. His mind is not tied to reality:
It is not the function of the poet to relate what has happened but what may happen – according to the laws of probability or necessity.
History tells us what actually happened; poetry what may happen. Poetry tends to express the universal, history the particular. In this way, he exhibits the superiority of poetry over history. The poet freed from the tyranny of facts, takes a larger or general view of things, represents the universal in the particular and so shares the philosopher's quest for ultimate truth. He thus equates poetry with philosophy and shows that both are means to a higher truth. By the word 'universal' Aristotle signifies:
How a person of a certain nature or type will, on a particular occasion, speak or act, according to the law of probability or necessity.
The poet constantly rises from the particular to the general. He studies the particular and devises principles of general application. He exceeds the limits of life without violating the essential laws of human nature.

Elsewhere Aristotle says, "Art imitates Nature". By 'Nature' he does not mean the outer world of created things but "the creative force, the productive principle of the universe." Art reproduce mainly an inward process, a physical energy working outwards, deeds, incidents, situation, being included under it so far as these spring from an inward, act of will, or draw some activity of thought or feeling. He renders men, "as they ought to be".

The poet imitates the creative process of nature, but the objects are "men in action". Now the 'action' may be 'external' or 'internal'. It may be the action within the soul caused by all that befalls a man. Thus, he brings human experiences, emotions and passions within the scope of poetic imitation. According to Aristotle's theory, moral qualities, characteristics, the permanent temper of the mind, the temporary emotions and feelings, are all action and so objects of poetic imitation.

Poetry may imitate men as better or worse than they are in real life or imitate as they really are. Tragedy and epic represent men on a heroic scale, better than they are, and comedy represents men of a lower type, worse than they are. Aristotle does not discuss the third possibility. It means that poetry does not aim at photographic realism. In this connection R. A. Scott-James points out that:
Aristotle knew nothing of the "realistic" or "fleshy" school of fiction – the school of Zola or of Gissing.
Abercrombie, in contrast, defends Aristotle for not discussing the third variant. He says:
It is just possible to imagine life exactly as it is, but the exciting thing is to imagine life as it might be, and it is then that imagination becomes an impulse capable of inspiring poetry.
Aristotle by his theory of imitation answers the charge of Plato that poetry is an imitation of "shadow of shadows", thrice removed from truth, and that the poet beguiles us with lies. Plato condemned poetry that in the very nature of things poets have no idea of truth. The phenomenal world is not the reality but a copy of the reality in the mind of the Supreme. The poet imitates the objects and phenomena of the world, which are shadowy and unreal. Poetry is, therefore, "the mother of lies".

Aristotle, on the contrary, tells us that art imitates not the mere shows of things, but the 'ideal reality' embodied in very object of the world. The process of nature is a 'creative process'; everywhere in 'nature there is a ceaseless and upward progress' in everything, and the poet imitates this upward movement of nature. Art reproduces the original not as it is, but as it appears to the senses. Art moves in a world of images, and reproduces the external, according to the idea or image in his mind. Thus the poet does not copy the external world, but creates according to his 'idea' of it. Thus even an ugly object well-imitated becomes a source of pleasure. We are told in "The Poetics":
Objects which in themselves we view with pain, we delight to contemplate when reproduced with minute fidelity; such as the forms of the most ignoble animals and dead bodies.
The real and the ideal from Aristotle's point of view are not opposites; the ideal is the real, shorn of chance and accident, a purified form of reality. And it is this higher 'reality' which is the object of poetic imitation. Idealization is achieved by divesting the real of all that is accidental, transient and particular. Poetry thus imitates the ideal and the universal; it is an "idealized representation of character, emotion, action – under forms manifest in sense." Poetic truth, therefore, is higher than historical truth. Poetry is more philosophical, more conducive to understanding than Philosophy itself.

Thus Aristotle successfully and finally refuted the charge of Plato and provided a defence of poetry which has ever since been used by lovers of poetry in justification of their Muse. He breathed new life and soul into the concept of poetic imitation and showed that it is, in reality, a creative process.