Two things, both of them natural, seem likely to have been the causes of the origin of poetry. Representation comes naturally to human beings from childhood, and so does the universal pleasure in representations. Aristotle, Poetics, IV. 1448b.
It is easy to assume that the ‘two things’ which Aristotle assigns as the ‘causes of the origin of poetry’ (aitiai phusikai) are specified in this passage. Poetry has two roots, one in our inclination to engage in imitation (IV.a5: to mimeisthai) and the other in a natural pleasure we take in representations (IV.a8: to chairein tois mimemasi).
So does this text specify poetry’s dual origin?
No, matters are not so straightforward. Aristotle explains that by means of imitation (1) we learn our first lessons (matheseis) and that (2) in seeing imitations even of things which are unpleasant we take pleasure because by means of these imitations we are ‘enabled to understand and work out what each item is’ (manthanein kai sullogizesthai).
Now (1) and (2) reduce to a single factor, taking pleasure through imitation in the acquisition of knowledge. Imitation – mimesis – yields elementary knowledge which doesn’t disclose the essence or eide of a thing but only its concrete or phenomenal properties. So what is the real second cause - aitia - of poetry? It is indicated, not in the passage above about representation, but clearly just down the page at 1448b20-21:
‘Representation, then, comes naturally to us, as do melody and rhythm (kata phusin de ontos hemin tou mimeisthai kai tes harmonias kai tou rhuthmou).
An instinct for melody and rhythm is then the second aitia of poetry.
Aristotle, Poetics, tr. A. Kenny, ISBN 10: 0199608369 / ISBN 13: 9780199608362 Published by Oxford University Press.
R. P. Hardie, ‘The Poetics of Aristotle’, Mind , Jul., 1895, New Series, Vol. 4, No. 15 (Jul., 1895), pp. 350-364: 354-355.
Jonathan Lear, 'Katharisis', Phronesis, Vol. 33, No. 3 (1988), pp. 297-326: 307f.