C. S. Lewis:
Every poem can be considered in two ways – as what the poet has to say, and as a thing which he makes. From one point of view it is an expression of opinions and emotions; from the other, it is an organization of words which exist to produce a particular kind of patterned experience in the readers. Another way of staying this duality would be to say that every poem has two years – its mother being the mass of experience, thought, and the like, inside the poet, and its father the pre-existing Form (epic, tragedy, the novel, or what not) which he meets in the public world. By studying only the mother, criticism becomes one-sided. It is easy to forget that the man who writes a good love sonnet needs not only to be enamored of a woman, but also to be enamored of the Sonnet. It would, in my opinion, be the greatest error to suppose that this fertilization of the poet’s internal matter by the pre-existing Form impairs his originality, in any sense in which originality is a high literary excellence. (It is the smaller poets who invent forms, insofar as forms are invented.) Materia appetit formam ut virum femina. The matter inside the poet wants the Form: in submitting to the Form it becomes really original, really the origin of great work. The attempt to be oneself often brings out only the more conscious and superficial parts of a man’s mind; working to produce a given kind of poem which will present a given theme as justly, delightfully, and lucidly as possible, he is more likely to bring out all that was really in him, and much of which he himself had no suspicion.
– A Preface to Paradise Lost