The most contentious issue in literary-critical theory, over the centuries, and especially with regard to poetry, is the debate about the didactic function of literature. The first work of literary criticism in English addresses the matter directly. In “An Apology for Poetry” (“apology” in the sense of an apologia, a defence of a belief, not an admission of guilt or regret), written around 1580 by that quintessential Renaissance man Sir Philip Sidney, and published posthumously in 1595, the poet-critic speaks of poetry’s “delightful teaching”. To distinguish it from and prioritise it over both philosophical and historical writing, his emphasis is…