A scholar with the gift of tongues who can smell a mouse in a text full of rats

In the confrontation between Hamlet and his mother, Gertrude, (Act III, Scene iv), when a sudden noise indicates that someone is eavesdropping, Hamlet exclaims: "How now? A rat?" and plunges his sword through the arras to kill the intruder, believing him to be his uncle Claudius, his father's murderer. Instead he discovers that he has killed Polonius, his fiancée's father.

Umberto Eco takes Hamlet's exclamation as an example of the difficulty in choosing the right equivalent for a seemingly simple word, rat, in various European languages, all derived from the Latin root rattus . The trouble is that these languages also have mouse, from the Latin mus . The obvious difference between the two rodents is size - a rat is larger than a mouse - but the nuances and connotations relating to them vary in different languages. So which one is right?

Eco proposes that in Italian, topo (mouse) is correct, for that is what an Italian Hamlet would say: " Cosa c'e? Un topo? " By contrast in Camus' The Plague , when one morning the protagonist, Dr Rieux, finds "a dead rat" on his doorstep, ratto must be used, for mice do not carry the plague, rats do. In both examples, and in translation generally, one does not rely on "definitions provided by dictionaries... but negotiates which portion of the expressed content is strictly pertinent in that given context". Thus all translation is negotiation, a give-and-take "process by virtue of which in order to get something, each party renounces something else".

Based on a series of lectures given in Toronto and Oxford, Mouse or Rat ? is a collection of essays on the difficulties, pitfalls, joys and rewards of translation. Eco draws on his own experiences as translator and editor, and on working with translators of his own work. As always he dazzles with his vast erudition, charm and humour. He believes that the translator's first "ethical obligation (is) to respect what the author has written... what he means". But what is meaning? Eco's definition is "that which remains unchanged in the process of translation". Thus, to translate an idiom, the translator must find an equivalent in the "destination language", and in so doing he may have to become "literally unfaithful" to remain "truly faithful to the meaning" of the original text. Eco gives examples from his own books that have baffled translators. He admits to sometimes playing linguistic games that are "ostensibly erudite" - a whole page listing the variety of tramps and vagabonds in The Name of the Rose , or allusions to a poem by Leopardi in Foucault's Pendulum , for which the English and German translators had to find equivalents in Keats and Goethe.

To illustrate the basic problem of "meaning", Eco submits the first chapter of the Book of Genesis in English to AltaVista, the internet search engine, asking it to translate the text into Spanish and back again into English.

The result is hilarious: "the spirit of God moved on the face of the waters", becomes "the alcohol of God moved on the face of the waters", for "AltaVista has merely a list of correspondences, like the Morse Code", and cannot "contextualise" or know that spirit is different used in a church from in a pub. So first there are differences between tongues, then between texts, "a very crucial point".

Of equal importance to meaning is context: "In order to translate, one must know a lot of things, most of them independent of mere grammatical competence", for "translation does not only concern words and language in general, but also the world, or at least the possible world described by a given text".

Sometimes translation changes and enriches the "recipient language", compelling it to "express thoughts and facts that it was not accustomed to express": the translations of American writers - Hemingway, Faulkner, Dos Passos - changed "the narrative styles of Italian and French writers after the War", while Heidegger's "radically changed the French philosophical style". Hence the need for "updating" certain key texts as language subtly changes over time. In the first line of Dante's sonnet to Beatrice - " Tanto gentile e tanto onesta pare... " - the words " gentile " and " onesta " had different meanings in the poet's time from today, and English translators have used various words over the centuries to convey the original intention.

Literary devices such as hypotyposis - "the rhetorical effect by which words succeed in rendering a visual scene" - and ekphrasis - when a verbal text describes a visual work of art - are straightforward, except with modern writers who, like Proust in A la Recherche du Temps Perdu , practice "occult ekphrasis": describing a work of art without giving clues. This has given Eco another opportunity "to play frequent games with paintings" - for example by Vermeer in The Island of the Day Before - hoping that "the cultivated reader" would have the thrill of recognising his sources.

Verse translation has the added problems of rhyme and rhythm. Eco illustrates the difficulty with English translations of The Divine Comedy : some translators have gone through endless contortions to keep the rhyme and the metre, while others have found the price "disastrously high", and achieved greater fluency without them.

When translation is from one medium to another it is called adaptation - as when a novel is turned into a film - or transmutation when a text serves as a basis for a ballet, or a symphonic poem, as in Debussy's Pelléas et Mélisande after Maeterlinck's play, and Prélude pour l'Aprés-midi d'un Faune , based on Mallarmé's poem. At their best "such transmutations serve to help appreciate the inspiring work better".

In conclusion, Eco returns to the Pentecostal "gift of tongues", reversing "the defeat of Babel". His own gift of tongues makes the reader of his brilliant book understand the enriching pleasure derived from the knowledge of languages. "When one learns a foreign language, one acquires a new country," Goethe said. Eco has made half a dozen countries his own.

Shusha Guppy is London editor, Paris Review .