I try to rejuvenate Cleanth Brooks's old thesis about the 'heresy of paraphrase.' This I do by analysing a couple of well-known poems and by performing thought experiments of the "possible world" kind. They show that paradigmatic examples of poems are not paraphrasable. A prosaic text can be improved with the aid of a paraphrase, but a typical poem cannot. The deeper explanation for the non-rephrasability of poetry is that our understanding of it is basically tacit. In this way I hope to give Brooks's original thesis a more solid foundation.