A birthday, and the reaching of a great age, make an appropriate time to celebrate a poet, and not any the less so if we plainly admit that poets are more often celebrated than read.

The Lithuanian-born Nobel prize-winner Czeslaw Milosz, who will be 90 tomorrow, was hailed by his fellow literature laureate Joseph Brodsky as "one of the greatest poets of our time, perhaps the greatest".

Yet he may well be better known - to the degree he is known at all outside the small circles of fellow writers and professors - for two works in prose.

The American critic Helen Vendler has called his Treatise On Poetry "the most comprehensive and moving poem of this half-century". Yet it waited 45 years for a full English translation.

Milosz has been a writer for more than 70 years, though for roughly 40 of those he was, as a political exile, largely cut off from the everyday life of his country, the voices of his countrymen and, indeed, the natural audience for his poetry.

Such exile can be fatal for a writer or it can, by the intensity of the acts of remembrance it requires and the degree of engagement with a language no longer daily heard, be oddly positive and productive. In the case of Milosz, who has seen at close quarters most of the worst that the 20th century had to offer, remembrance and truth-telling have become the main themes not just of his literary work across many genres but also of his significant other existence as a public intellectual.

Milosz was born in 1911 in Sateiniai, then part of tsarist Russia, to Polish and Lithuanian parents. Educated at secondary and university level in Wilno (now Vilnius), he was at the earliest age exposed to the sounds not only of his native Polish but also of Russian, Lithuanian, Yiddish, German and Belorussian, which were spoken by the various national groupings and social strata that lived cheek by jowl on the borders of the tsarist empire and what had once been the Polish commonwealth.

His youthful experience in Wilno of Polish religious and national intolerance, which generated constant tension and led to sporadic civil disorder in the city, inoculated him for ever against all theories based on blood, race or the natural superiority of "us" over "them".

Demagoguery, he wrote, "turned its sharp edge against foreign capital and 'Judeo-plutocrats', which ... the small-town lumpen proletariat took as a signal for assaults on peddlers' stands."

Some years later, ethnic hatred being many-sided, the aggressors of that time were to become victims when, during a temporary Lithuanian control of the city, those "who only recently had been beating Jews were now being beaten themselves for singing in Polish in church".

Milosz spent the war years in Warsaw, where he was active in underground literary circles. Many of his friends and fellow poets were killed by the Germans. Others, after the defeat of Nazism, were to fall victim to their Russian liberators.

As a member of the diplomatic service of the new "People's Poland", Milosz was posted first to the US, then to France.

With a growing standing as a poet and a reputation as a "progressive" left-winger rather than as a communist, he was at first a valuable asset to the regime. By 1951, however, with the cold war at its height and Stalin in his final phase of murderous paranoia, he had increasing reason to fear for his life, and defected.

So began a long period of exile that was at first lonely and impoverished: he was cut off from his literary peers and without means of support. The intellectual atmosphere of 1950s France was also strongly marked by pro-Sovietism, and an anti-communist literary intellectual did not receive the kind of welcome he might have a generation later.

He made friendships and alliances nevertheless, and his name began to appear in small literary-political journals alongside those of kindred spirits such as Albert Camus, Arthur Koestler, Ignazio Silone and Stephen Spender.

In 1953 he published The Captive Mind, the first of the prose works on which his reputation still rests. A study of the intellectual's accommodation to communism, it soon took its place with Koestler's Darkness At Noon and George Orwell's Nineteen Eighty-Four. Towards the end of the next decade, by which time he was teaching at the University of California, came his second prose masterwork, Native Realm, a memoir of his early life and study of "the other Europe" written for a Western audience he felt to be more full of vague goodwill than understanding.

Seen as the crowning achievement of Milosz's poetic work, Treatise On Poetry is a meditation in four cantos on war and destruction, poetry and poets and the obligations of art and the artist. In much of his other poetic work, however, he has avoided contemplation of the horror of his century in favour of what he calls the praise of tangible forms and the "direct hit with words".

It is, as one critic has said, "a poetry of mitigating circumstances", of ordinary things seen clearly: "The furriness of the beaver, the smell of rushes,/And the wrinkles of a hand holding a pitcher/From which wine trickles."

Though keenly aware of the poet's spiritual kinship with the Romantic revolutionary, ill with the insufficiency of existence and determined to "remake bread-eaters into angels", Milosz has eloquently explored in his reflective writings the dangers of that path and chosen for the most part in his verse to celebrate not the blood-red rose but the possibilities of transcendence and of human happiness, which "has the smell of freshly baked bread".