I have spent much of my life teaching, at very different levels. I began in the early 1960s at St Thomas's Secondary Intermediate School in the Ballymurphy area of Belfast, in front of a class of deprived and disaffected adolescent boys, many of whom would end up a decade later as active members of the provisional IRA.

I proceeded from there to work in a teacher training college, also in Belfast, and to spend time trying to persuade student teachers of the value of imaginative literature and other kinds of creative play in the educational process; I went on to lecture on poetry at Queen's University, and ended up in more recent years as a poet-in-residence at Harvard.

In each of these places, members of the audience differed widely in literary awareness, and in the degree of their assent to the idea that poetry was a subject worth discussing at all; I have known both the heckling of the have-nots at St Thomas's and the nods and gleams of the granny glasses in Harvard Hall; in each case there was a desire, repressed in the Belfast context but altogether ardent in Cambridge, a desire to have the worth and meaning of the art confirmed. What was at stake was the credibility of this honoured but hard to define category of human achievement called poetry. Even in Ballymurphy, those boys excluded by their social and cultural background from any contact with literary verse and disposed to regard it as some kind of fancy affectation, even they were curious despite their resistance.

There were plenty of influences at work to make them shy away: peer pressure, the macho conventions of the playground in a boys' school, a working-class shyness in the face of anything that smacked of middle-class pretension, but, even so, the mystery of the thing interested them and every now and again during those English classes something steadied and came into focus: for a concentrated moment the words they were attending to made sense and went home as only poetry can.

Another thing that happened during those English classes is also worth recalling. About once a week, and always unexpectedly, the headmaster of the school would suddenly appear at the classroom door. Mr McLaverty was a short-story writer of real distinction but he was also compulsively a teacher. He was meant to be in his headmaster's office all day, administering, but instead he prowled the corridors in his tweed suit and polished brogues, seeking whom he might interrupt in order to get in a bit of the actual schoolmastering that he missed so much.

"Right, boys," he would exclaim as he hurried across the floor to claim the boys for his own. And then, "Right, Mr Heaney!" in order to relieve me of responsibility for them, or rather to appoint me as his straight man in a double act which rarely varied.

"Mr Heaney," he would continue, "are they working hard for you?"

"Yes, Mr McLaverty," I would answer.

"And are you doing any poetry with them?"

"Oh yes," I would reply, "I am indeed."

"And are you seeing any improvement in them at all?"

To which the correct answer was, "Of course I am".

And then, climactically, he would turn his attention very deliberately from the class to me and inquire, "Mr Heaney, when you look at the photograph of a rugby team in the newspaper, don't you always know immediately from the look of the players' faces which ones of them have studied poetry?"

And dutifully, unfailingly, I would answer, "Yes, Mr McLaverty, I do know."

McLaverty would nod triumphantly and turn back toward the desks. "There you are now, boys," he'd say. "Work hard and don't end up down there with the rest of them, measuring the length of your spits at some street corner! Right, Mr Heaney!"

And away he would go in all his peremptory vigour, as memorable and problematical as poetry itself.

When I say problematical, all I mean is that poetry cannot be proved in the way a theorem can. McLaverty could only manage to get away with his proposition that poetry changed people perceptibly for the better because I was ready to connive with him. And anyhow, the boys in the class knew that the whole thing was a masquerade. But it is precisely this masquerade of fictions and ironies and fantastic scenarios that can draw us out and bring us close to ourselves. The paradox of the arts is that they are all made up and yet they allow us to get at truths about who and what we are or might be.

Mr McLaverty's caricature of the humanising power of poetry was tempting as well as comic because it was drawing upon two and a half millennia of western aesthetic and educational theory. From Plato to the present, from the Athenian academy to the parent-teacher meeting at your local primary school, there has been an ongoing debate about the place and the point and the choice of imaginative writing in the curriculum, and about the relevance of such material to the formation of the good citizen's sensibility and behaviour.

McLaverty's performance was in itself a kind of parody or exaggeration of one of the central ideas of this humanist tradition, the idea that there is an essential connection between the good and the beautiful, and that the study of the beautiful is actively conducive to virtue. This particular defence of the value of art was disastrously weakened in the last century, of course, by the historical fact of the Holocaust: what good is a devotion to and an appreciation of the beautiful, the question goes, if some of the most cultivated people in a most cultivated nation could authorise mass killings and attend a Mozart concert on the same evening? Yet if it is a delusion and a danger to expect poetry and music to do too much, it is a diminishment of them and a derogation to ignore what they can do.

What they can do is testified to not only by Mr McLaverty but also by Shakespeare's Caliban. In The Tempest, Caliban's description of the effect that Ariel's music produces in him could be read as a kind of paean to the effect of poetry itself. You remember the lines: Caliban is telling Stephano and Trinculo not to be worried about the mysterious tune that is coming out the sky above them and says:

Be not afeard; the isle is full of noises,
Sounds and sweet airs, that give delight, and hurt not.
Sometimes a thousand twangling instruments
Will hum about mine ears, and sometimes voices,
That, if I then had wak'd after long sleep,
Will make me sleep again.

"Sounds and sweet airs, that give delight, and hurt not": that, as a description of the good of poetry and of literature in general, will do. It is not required that the experience of the sounds change Caliban into another kind of creature, or that it have a carryover effect upon his behaviour. The good of literature and of music is first and foremost in the thing itself and their first principle is that which William Wordsworth called in his Preface to Lyrical Ballads "the grand elementary principle of pleasure", the kind of pleasure about which the language itself prompts us to say, "It did me good."

© Seamus Heaney 2002. This is an edited extract from an essay in his Finders Keepers: Selected Prose 1971-2001, published on Monday by Faber & Faber at £20. To order a copy for £17 plus p&p, call Guardian book service on 0870 066 7979