On 19 May 1425 Jan van Eyck entered the service of Philip the Good, and he made several confidential missions in Spain and beyond the confines of Christian Europe. However, almost nothing has survived of the commissions he carried out for Philip the Good. We know that he worked on the decoration of the Duke's residences at Hesdin (1431 or 1432) Brussels (1433) and Lille (1434). We also know that he painted portraits of Philip the Good and of his family, which have unfortunately been lost. Of his surviving works, only one is generally agreed to have been a commission for the Duke: an Annunciation painted for the chartreuse de Champmol. Yet although he was an official painter to the court, Jan Van Eyck was always free to work for other clients. It was in this way that he came to paint the Adoration of the Lamb which, according to the dedication inscribed on it, was consecrated on 3 May 1432.

Open wings, lower tier

This polyptych is mystical, not to say esoteric, in intention, and is imbued throughout with both spiritual and intellectual signification. When opened, it represents the communion of saints, which is "the new heaven and the new earth", in the words of the Revelation of St John. Thus the central panel of the lower tier portrays the saints symbolizing the eight Beatitudes gathered round the altar where the sacrifice of the Lamb is taking place, at the centre of the heavenly garden which has sprung from His blood.

To left and right, in the foreground, are two processions facing one another. One of these is made up of the Old Testament patriarchs and prophets, and the other of figures from the New Testament. Some of them are kneeling, barefoot. Behind them is assembled the hierarchy of the Church - popes, deacons and bishops, wearing sumptuous jewelry and clothes in the bright red of martyrdom. In the background are two further groups, facing each other as if they had just emerged from the surrounding shrubbery. These are, on one side, the Confessors of the Faith, tightly packed together and almost all dressed in blue; and on the other side, the Virgin Martyrs, holding out palm fronds and wearing in their hair crowns of flowers of a kind traditionally worn by young girls at certain holy ceremonies. In the middle of the panel, around the altar where the Lamb spills forth his blood, angels kneel, holding the emblems of His Passion. Grace is symbolized by a radiant dove hovering in the sky, and eternal life is represented by a fountain in the foreground. A paradisiacal landscape (see in a new window) runs across all five lower panels, uniting them in a single composition. It is strewn with plants from different countries and flowers of different seasons. The central panel is vibrant with green, while those to the sides are more arid and rocky. The horizon sits high in the frame and is closed off by groves of trees, behind which clusters of fairy-tale buildings can be made out, representing the heavenly Jerusalem.

The community of saints also extends onto the side panels. Magnificently arrayed horsemen, representing the Soldiers of Christ, are followed by the Just Judges. Opposite them are the Holy Hermits who have renounced the world, and the Pilgrim Saints, who were favourite figures of identification throughout the Middle Ages. They are led by a giant of a man, St Christopher. Many later commentators have suggested that his great height would have reminded the contemporary viewer of Jodocus Vyd's brother, also called Christopher.

Open wings, upper tier

In the middle of the upper tier is God Almighty, the Word, essence and origin of the universe. He is dressed in red and is crowned with a magnificent tiara. On his left is Mary and on his right, St John the Baptist. These central figures are surrounded by angels who are singing or playing instruments. At the far right and left of the composition respectively are the figures of Adam and Eve. They were painted by Jan Van Eyck, and are set into trompe-l'oeil niches. Light and shadow play delicately over their forms which stand out as though they had been sculpted in the round.

The realism of these two figures struck contemporary viewers forcefully. The representation of everyday life in close proximity sacred elements can be interpreted both as a new aesthetic and as evidence of a more populist approach to Christianity. We can see Adam and Eve, the first couple, whose posture and subtly rendered nakedness make them seem more actual than the main subject, which takes up several panels. One might even describe these panels as scenes in the story of Adam and Eve because (from the standpoint of the observer) they are shown slightly elevated and walking towards the holy events. The undisguised pregnancy of Eve, the offering of Abel and Cain and Cain's murder of Abel shown above Adam and Eve, respectively, open the route to humanity's suffering - but also to God's grace and his willingness to sacrifice his own son, the "Lamb of God," in order to fulfill his promise of redemption. This interpretation takes into account the way in which the artist's aim to paint realistically accorded perfectly with the profound piety of his age.

Closed wings

The realistic style of the figures of Adam and Eve continues on the outside of the panels when the altarpiece is closed. The external decoration shows the Erithraean and Cumaean Sibyls, Prophets Zacharias and Micheas, the figures of Jodocus Vyd, the donor, and his wife Isabelle Borluut kneeling on either side of two grisaille (painted in gray to resemble statuary) representations of St John the Baptist and St John the Evangelist, and the Annunciation with the angel Gabriel and the Virgin Mary. The angel and the Virgin of the Annunciation (which occupy the same panels as Prophets Zacharias and Micheas, respectively) are separated by two small panels, one with the representation of an arched window looking out upon a city square, and the other with a wash basin and ewer set into a niche and a white towel hanging from a rail beside it.

A striking feature is the disparity in the scale of the various figures: no less than four changes of scale exist of the outside of the wings. There are also disparities in approach; some parts are almost prosaically factual, others almost visionary in approach. Three orders of reality are present: a narrative representation of a sacred subject (the Annunciation), two highly factual donor portraits and two simulated sculptures. Yet there is a strong attempt to impose a uniform framework on these disparate elements through the governing factor of the light, which falls uniformly in all the panels from the right, and also through the use in the upper panels of a beamed ceiling running through the whole scene, and, in the lower panels, of the same cusped trefoil arches to frame the figures.

We must be careful not to exaggerate the significance of the realism of this altarpiece. It in no way represents "progress", whether technical or aesthetic, in the means of representation. Its increasing use should rather be attributed to a change in the theological motives of the age. During the Middle Ages, official doctrine had placed earthly realities on the lowest level of the scale of Creation - if they were not, indeed, the work of the devil himself. However, by the time of the Van Eyck brothers, religious thought had begun to absorb the influence of the great mystics. People began to view the entire world as the work of God, the source of all creation, and present in its every detail, no matter how small and insignificant. Thus nature came to be seen as sacred, as it was a reflection of God's spirit. Where mediaeval art had focussed on a world beyond this world, the new art was devoted to scrupulous observation of what lay before the artist's eyes. Imagination was replaced by attention. Every creature, every thing, was now perceived as a sign - a metaphor - representing a spiritual truth. This vision determined the artist's vocation: to imitate the visible world as faithfully as possible, not merely in order to glorify creation, but so as to reveal the metaphysical dimension that lay concealed within.