In The Little Book of Plagiarism, Richard Posner excerpted T.S. Eliot’s famous comment about poetic imitation. I tracked it down and give a bit more of the context here. From Eliot’s essay “Phillip Massinger” in The Sacred Wood: Essays on Poetry and Criticism:

Reading Shakespeare and several of his contemporaries is pleasure enough, perhaps all the pleasure possible, for most. But if we wish to consummate and refine this pleasure by understanding it, to distill the last drop of it, to press and press the essence of each author, to apply exact measurement to our own sensations, then we must compare; and we cannot compare without parceling the threads of authorship and influence…

One of the surest of tests is the way in which a poet borrows. Immature poets imitate; mature poets steal; bad poets deface what they take, and good poets make it into something better, or at least something different. The good poet welds his theft into a whole of feeling which is unique, utterly different from that from which it was torn; the bad poet throws it into something which has no cohesion. A good poet will usually borrow from authors remote in time, or alien in language, or diverse in interest.