When Jonathan Miller's tall figure looms into view he begins talking straight away, as though resuming a previous conversation. His wife has been bagging leaves in the garden when I arrive. Go in, she says - he's expecting you. He has, after all, phoned several times to check that I am coming. I was expecting to be bulldozered by his formidable intellect, but this is obviously not going to be a formal interview: we simply wander, chatting, into the garden to take a look at the metal constructions that will form part of his show at the Boundary Gallery. He was taught to weld in Santa Fe, he says, where he was directing opera. He toured the local junkyards in a pick-up truck, then stripped to the waist and got down to work: a touch of Chillida here, a smidgen of David Smith there. "Of course, as you can see," he insists, "I'm a formalist, an old-fashioned modernist. I don't have much sympathy with postmodernism. Some people would probably say that what I make is derivative but all great artists - not that I would call myself an artist, more a putter-together of this and that - are influenced by others."
Miller, despite his ageless appearance, is now pushing 70 and much less manic and more modest than I had expected. Yet his enthusiasm for all things philosophical, aesthetic and scientific is as intense as ever. "Please don't call me a Renaissance man," he says. It's such a contemporary view to think it odd if one is passionate about a wide range of subjects. Yet his conversation is littered with verbal "footnotes" as his mind races tirelessly from topic to topic; from Shakespeare to opera, from the unconscious and psychiatry to anthropology and his dislike of contemporary French philosophy (he is an admirer of the Anglo-American school), stringing them all together as an accomplished composer might handle a complex melody or a conductor the different instruments in an orchestra. He still has the same intellectual curiosity that led him, as a young boy, to discover the delights of biological symmetry, which set him on the course to study medicine.
It was Beyond the Fringe, of course - that glittering revue of irreverent satire in the early Sixties - that brought him to public notice. Since then he has directed theatre, TV and opera from New York to Florence, written and presented a series on the history of medicine and was the executive producer of 12 of the BBC Shakespeare series. There was also his delightful film of Alice in Wonderland. Wasn't all that more than enough for one man? Why the art? Well, he has always made art. His father, a psychiatrist, was also a sculptor, his mother a writer. He was brought up surrounded by books and paintings, many of which still fill, to bulging point, the Camden house that he has lived in for years. The walls of the narrow staircase are crammed with prints and etchings: Piranesi, Greco-Roman columns and pediments, Muybridge's photographs on the analysis of movement. "You see, I just like form," he says, as we climb another flight to look at paintings in the bedroom, photographs in the loo.
He also makes collages from bits of detritus. Mostly shreds of advertising posters scraped off walls in New York or Italy. When rehearsing, say, Tosca, he might be found during the siesta hour peeling choice samples from local hoardings. What he is interested in is the incidental, what is passed by. His art, if it is about anything, is about making the negligible visible. What Constable confessed to as a love of "old rotten banks, slimy posts and brickwork". He talks of the pleasures of fiddling with bits and pieces, though his is a highly informed aesthetic. While at Cambridge, amid the Footlights reviews and the medical exams, he found time to take in a good deal of art history. He draws on Kurt Schwitters, on Joseph Cornell and Braque. But the results are very much Jonathan Miller; eclectic, idiosyncratic, like those of a highly visual and literate magpie.
He is in the middle of filming a programme for television on atheism. Did he, then, consider art to be the thing that filled the God-shaped hole in contemporary society? Did it perhaps provide the only possible route by which, in a post-Nietzschean world, we might momentarily encounter the metaphysical or the sublime? And then he was off again like a foxhound that had sniffed its quarry. It was all to do with the expression of human co-operation rather than anything mystical. Co-operation leads us to have empathy with one another. Or to use the word he prefers, the word used by the philosopher Adam Smith, sympathy. But wasn't that just too mechanistic a view to explain how we feel when we hear Beethoven's "Eroica" or Bach's St Matthew Passion or a speech by Shakespeare? And he starts to talk of his love of King Lear, which he has directed many times, and as we sit in his homely kitchen drinking coffee at his long kitchen table, next to the wall covered with children's drawings, photographs of him with the young Alan Bennett et al, his children and grandchildren, he quotes Lear, who, when half out of his mind, turns to his daughter and says, "I think this lady to be my child Cordelia," and breaks down in tears. It is a moving moment. He is genuinely affected and takes time to compose himself. This is a man with an enormous mind. Yet at this minute, I can't help feeling that he is wrong - that what he had just experienced is more than a highly sophisticated evolutionary response.
So we have another go at a definition, after he asks, with great courtesy, if I mind if he smokes. Is what he has just felt equivalent to what the poet Wilfred Owen called "pity"? Yes, that's something like it. But it is a human pity. I ask if he accepts Melanie Klein's notion of art as a form of "reparation". This is a theory he rather likes, and later, as I am leaving, he tells me a funny story about an interview he did with Hanna Segal, the Kleinian psychoanalyst, on TV - it is easy to forget in all this cultural chat that he has a great sense of humour - though he doesn't think much of Freud. Freud just got the unconscious wrong, he says. He prefers the views of the cognitive behaviourists; the unconscious not as a dark vault full of secrets, but as an "enabling unconscious", like a computer, where the desktop is too small to keep everything needed on it, so thoughts and information are stored in files and folders that can be accessed when necessary.
I try to bring him back again to art. Really it is shape and form that please him. He pops upstairs to bring down an antique cobbler's last and an antique wooden beret stretcher and takes great pleasure in showing me how form has followed function. He talks of the delight of making, how we deeply underestimate the pleasure of doodling, and play. That's what artists are good at, and through play they are able to take time to notice what is incidental and place it for a moment in the centre of the frame. Think of Auden's great poem "Musée des Beaux Arts". How Brueghel places Icarus, falling from the sky, at the edge of his picture when the main thrust of life, the ploughman ploughing, the ship sailing, seems to be going on elsewhere. Breughel makes us aware of the previously overlooked. Art can do something like that.
Metal, wood and paper constructions, Boundary Gallery, 98 Boundary Road, London NW8 (020-7624 1126), 26 September-1 NovemberReuse content