Poetry is a subject hardly less difficult of definition than Religion, into which, indeed, in its higher reaches, it runs up. There has, in particular, been great difference among philosophers and critics as to whether metre is one of its essential elements. To Aristotle and Plato rhythm was of subordinate importance, and among modern philosophers Coleridge explicitly declares that there may be poetry (as distinct from a poem) without metre. The set of more recent authority, however, appears to be in the other direction. Etymology undoubtedly favours the more comprehensive definition, for among the Greeks poietes signified "a maker," "a creator," not necessarily of song, nor even of any other work of art. A consideration of much greater weight is that the opposite view implies the equal importance of form with contents, since it is admitted that there is much prose which lacks no quality of the highest poetry save that from its structure regular rhythm is absent. Nor even in form is the difference between verse and prose-poetry absolute; for the latter almost invariably falls into a rhythm of its own - less regular than that of verse, but none the less yielding abundant pleasure to the ear by its mere movement. To say that there can be no poetry without metre is to assert that there is no poetry in Hebrew literature, which, though it has no nearer approach to rhythm than parallelism, nor to rhyme than assonance, has yet produced the Book of Job, the Prophecies of Isaiah, the Psalms, the Canticles, and the Song of Deborah. On the whole, therefore, it seems preferable to define poetry as the art in which imagination, fancy, and emotion have for their medium derated language, sometimes rhymed, and generally metrical. By Coleridge the origin of metre is traced to a spontaneous effort in the creating mind to hold in check the workings of passion, and so to ensure it against the evil of unrestrained expression. This, however, is to assume that the creating mind achieves no more than it intends. It is excessively difficult to see how an aim so remote, and calling for so resolute an exercise of the will, could fail to be present to the poet's consciousness, whereas it is matter of universal experience, as indeed Coleridge concedes, that the expression of imagination and passion in language more or less rhythmical is purely instinctive. It might with as much, or as little, plausibility be suggested that when the poet adds to his metre rhyme, he does so in order further to increase his difficulties, and not because of the enhanced pleasure which he derives from the musical terminations of his verses, and wishes to communicate to his reader.
Poetry is divided into three main categories - lyric or subjective, including the song, the hymn, the elegy, the ode, the sonnet; epic or narrative, to which the ballad belongs; and dramatic. In a single word, these categories may be expressed as Song, Tale, Play. It is not easy, however, to find a place under either head for didactic and satirical poetry. A large part of the production in both these kinds occupies neutral territory between poetry and prose, being prose as to its contents and poetry as to its form. It may be worth consideration whether, having regard to the tendency of the times to analytic and philosophic poetry, the time has not come to erect a fourth category - the reflective - which would relieve the epic and the lyric of much that does not naturally fall within the province of either. The monologue, a form which Mr. Browning did so much to develop on the dramatic side, is often at once lyric, epic, and dramatic.