ONE of the greatest of paintings in the Western tradition is the so- called Maestà of Duccio di Buoninsegna, commissioned on 9 October 1308 for the high altar of the cathedral of Siena. The work took more than 32 months to complete, and consists of a large image of the Madonna enthroned with the Christ Child and surrounded by the whole company of heaven. On the reverse are 26 surviving panels that tell of the Passion, death, and resurrection of Christ.

When completed, the altarpiece was taken in solemn procession through the city on 9 June 1311 and installed with great ceremony. It remained there until 1515, when it was moved to a side altar, and in 1717 it was cut up into seven parts and gradually dispersed. Individual panels are now to be found in collections in London, New York, Madrid, and elsewhere.

We know that generations of artists, including Michelangelo, journeyed to Siena not so much to pray in front of the Virgin's image as patroness of the city as to learn from Duccio and to see for themselves this extraordinary monument to human craft and skill. Since 1878, the principal part of the altarpiece has been in housed in a museum. Still the pilgrims come, seeking it out.

The Irish-born Roman Catholic and contemporary artist Sean Scully first saw it in 1984, the year that a son of his died. He has been painting triptychs ever since, but, whereas Duccio's painting and that of many influenced by him had been figurative, Scully has abstracted both the image and the form, so that the essentials of the threefold nature of the triptukhos with a powerful central panel is all that we see.

The Pallant House exhibition is helpfully staged in three identical cube-shaped rooms that, side by side, reflect on the patterning of threes inherent in the shape of a triptych. For Scully, the significance of three goes beyond Christianity, and he says: "I love the idea that the triptych is connected to religion in the Christian sense, but also to other religions and to the number three that is special in so many ways."

Gone are the attendant patron saints. You cannot find Ansanus, Savinus, Crescenzius, or Victor here. There is no hint of the Virgin.

But there is a strong prevailing sense of our being brought into a presence, of finding a prevailing peace within the mysterium. Any visitor who is conscious of the sculptures of the late Edward Robinson, or who has attended a retreat led by the wood-carver priest Canon Christopher Lewis, will recognise what is going on at once.

Back in the 1960s, Scully had been employed in a print workshop, and his overlaying of images owes something to that training, as he is keen to explore textures that run the length of a line.

Whether he is painting on wood, canvas, or metal, his paint layers are built up in a way that suggests a lithographic print process. One image is laid on another so that on closer inspection what at first seems to be a single colour proves to be severally composed.

This deeply engaging exhibition goes back before that moment of encounter with the Duccio, and begins with a series of closely observed monochrome lines that reflect his familiarity with the work of his American teachers at Harvard, Carl Andre and the minimalist Robert Judd.

After a gradual shift to colour (and, in 2008, 12 Triptychs progresses from black-and-white to colour, like an old television being turned up), the retrospective culminates with his latest piece. Painted on three square linen canvases, Arles-Abend-Vincent, still smelling of oil and linseed, intrudes vertical and horizontal blocks of colour around a central square, triptychs within triptychs, and a palette that looks back to Van Gogh as much as to Duccio.

"Sean Scully Triptychs" is at Pallant House Gallery, 9 North Pallant, Chichester, Sussex, until 26 January. Phone 01243 774557.

www.pallant.org.uk