The language of poetry is usually rhythmical. Yet so is the language of lots of prose. In fact, this is probably the point to confess that almost everything we imagine peculiar to poetry — metre, rhyme, imagery, symbolism, heightened language, impassioned emotion —can be true of prose as well. Milton treats words as valuable in themselves, but so does Virginia Woolf.

Admittedly, not all prose is like this. The language of the Times Business section is not usually awash with symbolism. But nor is all poetry. Some modern verse (Philip Larkin, for example) is deliberately couched in an everyday, prosaic idiom. And some everyday idiom could be mistaken for poetry — which was how the playwright J. M. Synge considered the talk of the peasantry in the West of Ireland.

So what is the difference between poetry and prose? Some critics have concluded that the only watertight distinction is that in poetry the writer decides where the lines end, whereas in prose the typesetter does. Line-endings in poetry matter in a way that they don’t in prose. Take these lines from John Keats’s To Autumn: And sometimes like a gleaner thou dost keep Steady thy laden head across a brook . . .

F. R. Leavis, one of the greatest English critics, claims that as we pass from the end of the first line to the beginning of the second, we feel the precariously balancing movement of the gleaner as she steps from one stone in the stream to another.

Prose can exploit rhythm, too. Consider the splendid opening sentence of E. M. Forster’s A Passage to India: “Except for the Marabar Caves — and they are twenty miles off — the city of Chandrapore presents nothing extraordinary.” The sentence is discreetly but magnificently well-pivoted around the second dash, as the first two sub-clauses counterbalance the last two. There is even a subliminal hint of metre. Technically speaking, you could read the sentence as four iambic trimeters: di-dum-di-dum-di-dum. The whole line is a masterpiece of mildly bored, very English sang-froid.

Some poems make a terrible racket: Mine eyes have seen the glory of the coming of the Lord: He is trampling out the vintage where the grapes of wrath are stored . . . By the shores of Gitche Gumee, By the shining Big-Sea-Water, Stood the wigwam of Nokomis, Daughter of the Moon, Nokomis . . .

The metre of these lines, by Julia Howes and Henry Wadsworth Longfellow respectively, is so thumpingly regular that it cramps the reader’s style. As with nursery rhymes, it leaves the speaking voice no freedom to improvise. The metre determines how we read the line — not the case with “Thou still unravished bride of quietness”, or “To be or not to be, that is the question”.

Lines like these are more like an orchestral score than a rigid set of instructions. The voice or the eye can bring them alive, play them in different ways. Two actors may inflect them with quite different cadences. Like pianists, their performances of the same piece can differ considerably.

Contrast By the Shores of Gitche Gumee with these lines from Sir Walter Raleigh: As you came from the holy land Of Walsingham Met you not with my true love By the way as you came? How shall I know your true love That has met many a one As I went to the holy land That has come, that has gone?

Every line of this modest little verse springs on the reader a new sort of rhythmical surprise. Note in particular the stroke of genius by which the word “Walsingham”, with its delicate lilting, becomes almost a line in itself. The final line, “That has come, that has gone”, is a sort of dramatic staging of the very thing that it talks about, as the two finely balanced clauses, separated by a slight pause, enact a sense of toing and froing.

“That has come and that has gone” would be much less effective rhythmically.

Rhythm runs deeper in us than we are aware. It is one of the most primitive of poetic features. It is a matter of the body and its biological cycles — of the heaving of the breath and the pulsing of the blood. It is one of several ways in which poetry returns us to our infancy, in which all we know is stresses and impulses, not meanings. Poetry is a way of putting us back in touch with the deep rhythms of the body, rerooting words in the bodily life from which they spring.

But it does so at the level of meaning, not just instinct — because rhythm is also a matter of patterns and resemblances that are lodged deep in the mind. It is the background music of our daily speech. What poetry does is to turn up the volume.

How to Read a Poem by Terry Eagleton

Blackwell (£9.99, offer £9.49 inc p&p from 0870 1608080)

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