Like rhyme, rhythm, tone and imagery, ambiguity is as much a part of everyday language as it is of poetry: “Refuse to be put in this basket”, for example. Or the sign “Way out”, as read by an American. Some publicans have now taken to casting doubt on the authenticity of their products by advertising “real” ale.

Poetry, however, is the most ambiguous language of all. This is because a poem is the most information-rich piece of communication one can imagine. A line of verse such as “Eyeless in Gaza, at the mill with slaves”, from Milton’s Samson Agonistes, says more than “Have I missed the last bus?” because it forces us to attend to the way that every word is bound up with another.

This also happens, among other things, at the level of sound. The “z” of “Gaza”, for example, picks up the “s” of “Eyeless” and passes it on to the first and last letters of “slaves”. This is what we mean by a poem’s texture – the way in which its sounds and rhythms are woven into an opulent tapestry of sense.

This interweaving happens at the same time at the levels of meaning, connotation, symbolism, metre, rhyme and so on. All of these different dimensions can conspire together. Perhaps the hissing effect of Milton’s mighty line catches up something of Samson’s lofty contempt for his degrading surroundings, and so contributes to the poem’s psychological truth.

Poetry, then, is the type of language in which every unit is dynamically bound up with every other. Tone, meaning, sound, metre, rhythm and so on, are interactive systems which constantly modify each other.

We could, in fact, read “Have I missed the last bus?” in just the same way – which shows that poetry lies as much in the eye of the beholder as it does in the words themselves. The final “s” of “bus” picks up the “ss” of “missed”, perhaps with the effect of an exasperated sigh.

But poetry is the type of speech that, by thrusting the materiality of its words upon our attention, makes us especially sensitive to these effects. It is striking that from the poet’s viewpoint much of this immensely intricate activity goes on quite unconsciously. It is a matter of knack and intuition, of being inside language, rather than calculatedly manipulating it. It is more like returning a serve in tennis than building a cowshed.

This is why poetry is so often ambiguous. By placing every verbal unit in dynamic relation with every other, it generates such semantic complexity that it allows for the maximum degree of interpretation. When Shakespeare writes in his sonnets: When my love swears that she is made of truth I do believe her, though I know she lies we can either read it literally, or as meaning: “When my love swears that she is truly a virgin, I do believe her, even though I know she has sexual intercourse.”

Because this is art, however, the line can very well mean both things simultaneously. As Oscar Wilde remarked, art is the place where one thing can be true, but also its opposite. Poetry puts the law of noncontradiction into suspension. There are so many things you can have in poems that you can’t have in real life – a smile without a cat, a cathedral built of marzipan, a square triangle, a boy who is also a banana – poetry testifies in its wealth of possibilities to the fact of human freedom. It is language liberated from the actual and sent out on a spree. Why should language conform itself obediently to reality? Why shouldn’t it create it instead?

William Blake’s Tyger poem is a magnificent instance of ambiguity: Tyger! Tyger! Burning bright In the forests of the night, What immortal hand or eye Could frame they fearful symmetry?

The tone is awestruck, even reverent, full of slightly breathless admiration. Yet what do we make of this, just a few verses later? What the hammer? What the chain? In what furnace was thy brain? What the anvil? What dread grasp Dare its deadly terrors clasp?

We know that Blake, writing at the time of the Industrial Revolution, was a radical who hated industry and machinery; yet the speaker of the poem is using exactly such imagery to portray the tiger. It is as though the beast has “Made in Manchester” stamped on its belly. It may be that the poem is both praising the tiger and satirising the speaker’s pathetically inadequate attempts to understand this sublime source of energy.

Poetry holds conflicting attitudes together, without necessarily resolving them. What did Blake really think of the tiger? We just don’t know. Neither can we ask him – and even if we could, we would not necessarily accept what he said. How to Read a Poem is published by Blackwell (£9.99, offer £9.49 inc p&p from 0870 1608080)

Times Recommends

• The Sunday Times 100 best holiday reads
• What will we be reading this summer: the booksellers predict
• Literary festivals: a survivor’s guide