Everyone thinks he can write a song, and now I have. The haunted, wild look you see in the eyes of professional songwriters at cocktail parties rises from the popular idea that any of us, given the chance, can do what they do as well as they do it – the fur earmuffs you see them wearing even on the hottest summer days are there, too, from a desperate attempt not to have to listen to the songs the rest of us write and insist on playing for them. “I think I just feel how everyone feels, that I have three or four really great folk albums in me,” Hannah says complacently on the US television comedy Girls. Everyone thinks that.

But now I have done what all of us amateurs dream of doing – written words to someone’s music and heard it sung. Not a song alone, but an entire work – called (when I am in a puffed up mood) an oratorio, and when I am feeling more modest, a mere song cycle, and when I am feeling more modest still, a concept album, as yet unrecorded. It’s called Sentences, with music by the inspired young composer Nico Muhly, and seeing it premiere a few weeks ago, at the Barbican Centre in London goes high on my list of things I did that made my life matter – like the birth of a child, only with less sweat and better dressed.

The story we told was that of the great computer scientist Alan Turing, and it was sung by the amazing counter-tenor Iestyn Davies – but I won’t detain you with its genesis or my own sense of what in it works and what doesn’t. I will say that I was working on the libretto for the song cycle even as I was also writing the words for a musical comedy, properly so-called, and I discovered that in work meant or dreamed of for the commercial theatre, every syllable gets argued over. Is the emotional logic entirely lucid? In our Barbican-directed oratorio a great deal of indirection and obliquity was welcome. In our age, I’ve decided, the difference between entertainment and art is that in entertainment we expect to do all the work for the audience, while in art we expect the audience to do all the work for us.

But the deeper relation between words and music – the way they land in the listener’s ear, and then her soul – is more complicated than it seems. Music alone is puzzling enough – how it is that the mind makes sound into music and music into meaning is one of the big unanswered questions. No matter how hard we craft them for lucidity and shape and dramatic clarity – and it’s the good faith of the librettist’s art form to do so as elegantly as he can – music and words together exist in the end in an older realm of magic and enchantment, a place where the nursery rhyme and the church hymn and the pop single all meet. They work as spells do – that is, either entirely, or not at all. We sing and the magic door swings open, or it doesn’t, and there’s no explaining it. Three boys from Liverpool sing “She loves you, yeah, yeah yeah ” and the world turns off its axis. Had they sung, as Paul McCartney’s father wanted, “yes, yes, yes”, the old path would not have changed.

Yeah, yeah. yeah. The Beatles in concert, 1964

The libretto writer, I should add at once, is merely the junior partner in the enterprise – or not even a partner, more like the man who sweeps out the candy wrappers from the theatre floor after the patrons leave. Who now remembers the name of the man who set the text for Handel’s Messiah? Well, it was Charles Jennens. The only libretto writer whose name anyone remembers – other than the great lyricists of the American musical theatre, the sacred law firm of Mercer, Loesser and Hart – is Lorenzo da Ponte, who is my hero. He was Jewish and a priest, and a Venetian and a New Yorker. It’s a sympathetic package, and he wrote for – more than “with” really – wrote for Mozart, the three operas that may well be the height of all artistic creation: The Marriage of Figaro, Cosi Fan Tutte and Don Giovanni.

Lorenzo Da Ponte 1749-1838

• Born Emanuele Conegliano, to a Jewish family in Venice. converted to Roman Catholicism in 1764 and was baptised with the name Lorenzo da Ponte
• Ordained as a priest in 1773 but nevertheless fathered two children, and was subsequently banished for 15 years from Venice
• Moved to Vienna where he scored Mozart’s best-known Italian operas – The Marriage of Figaro (1786), Don Giovanni (1787) and Cosi Fan Tutte (1790)
• Later moved to London and then the US where he ran a grocery store, gave Italian lessons and was eventually appointed professor of Italian literature at Columbia College

If the aliens arrived on earth from Neptune and asked me “what should we go out to see first that you humans have made?”, I think I would say Cosi Fan Tutte. (Though I also think I would add “Oh – and watch Cary Grant in North By Northwest” – but then the aliens would say, “Yeah: we saw that already on Neptune. Everybody in the galaxy has seen North By Northwest.”)

But a single touch of contrivance spoils it all. Any time we feel the authors creating coincidence or engineering emotion, making melodrama rather than musical drama – shoving incidents around rather than exploring character in collision with itself or another – we rebel inside. In Cosi Fan Tutte we accept the convention of disguised Albanians. But we accept it because Mozart writes his most sublime music for the silliest parts. If it sounded cute, we would rebel against it. That “yeah, yeah, yeah” mattered because it was exactly what such a boy would actually say to a friend about a girl. The smirk, or the hack’s weary knowing devices, are both enemies of enchantment and, without enchantment, music and words together mean nothing.

A 2009 production of Mozart’s Cosi Fan Tutte, performed at the Salzburg Festiva

Music is so emotionally overwhelming that it pushes the discursive and explanatory roles of language aside – and it is part of the job of the libretto writer to get out of its way. Even in Handel’s Messiah we recall lyrical fragments more than whole stanzas. “Unto us a child is born, “How beautiful are the feet…” “All we, like sheep”. When we think through our experience of our favourite oratorios, our most important pop songs, our favourite operas, they are almost always the experience of a forceful fragment – three or four words – “How beautiful are the feet”, or “Shake it off”.

It is a mysterious, semi-physical response, in which the audience does as much work as the artists. It works or it doesn’t. Small fragments of sound and sense strike our hearts as shrapnel strikes our skin. They lodge and wound us, independent of their intended trajectory. The audience responds or it doesn’t. The audience is less like a crew of supercilious analysts and more like a magnet set to one pole or the other. If the pole is right, the audience is drawn irresistibly to the sound on stage. If it isn’t, no amount of seduction or intelligence can draw them in, any more than a physical magnet can be made to adhere to metal by good will or affection. Sung words belong more fully to the world of ritual and routine, of incantation and mother’s murmurings, than to the fully lucid and well-lit world of argument. The words work or they don’t.

Sondheim’s “Send In The Clowns” is sung by Desiree (Sigrid Thornton) in this 2009 production of A Little Night Music.

The one thing I have learned through the process is this – our minds make meaning out of music by not making too much meaning out of it. One learns as a librettist to tiptoe to the edge of argument, and then back off to the limbo-land of implication and indirection. The most popular lyric of Stephen Sondheim is, after all, the most offhand – a rueful farewell, but exactly to whom or exactly why we don’t always know. I hear the producer or the scriptwriter asking “Send in the clowns? Shouldn’t it be call off the clowns?” But in the clowns must come for reasons only clowns and composers know.

I have not learned why music matters most – but I have learned a great deal about the power of voices, the limits of language to insist and its power to invoke, and about the mysterious magnetism that passes between an audience and its art. Above all, I have learned that musicians are a superior race. We are lucky to share this planet – or any other – with them.

• Adam Gopnik has lived in Paris and wrote the book Paris to the Moon

A Point of View is usually broadcast on Fridays on Radio 4 at 20:50 BST and repeated Sundays 08:50 BST