When the basilica at Chartres was ravaged by fire in 1194, the town needed a new church. But what arose from the ashes was a monument to a new way of thinking - about God, the universe, and humankind's relation to them both.

The nave of Charters

Chartres cathedral was the first of the classic Gothic churches, and its design set the standard for the cathedrals in nearby Rheims and Amiens: the foundations stone of the latter was laid in 1220, just as the high vaults of Chartres were being closed.

It is not enough to speak of Gothic as an architectural style. The builders of the Middle Ages did not and could not draw, as we do today, from a palette of aesthetic but ultimately arbitrary choices. They could dispense with established ideas and techniques only by revising the entire conceptual basis of their world view. And because the church was the hub of society, and religion its governing principle, the appearance of a new way of building cathedrals reveals a new concept of what it means to exist in the world.

Such seismic shifts in thought cannot happen without the conjunction of many forces. In particular, these revolutions cannot be enacted solely in the mind - for Chartres is made of stone and glass, and could not have been realised without the craftsman and the engineer, and the development of technologies that can make the abstract concrete. Who were the workmen and the architects who made Chartres, and how did they know how to do it? Where did their materials come from? How does one supervise a construction project that takes a generation or more? And how, hundreds of years before the mechanics of Galileo and Newton, does one lift great stones to the high heavens, and be sure that they will not come tumbling down again around your ears?

In this book we shall see how the architects of the Gothic cathedrals - men like Villard de Honnecourt, who devised water-powered saws, perpetual-motion wheels, fabulous drinking birds and hand warmers to comfort bishops during a chilly Mass - were medieval Leonardos, ingenious inventors with a keen eye for nature and an innate artistic sensitivity.

Any major innovation in the Middle Ages needed social and theological justification, and was seldom possible without a battle being fought between radicals and conservatives. This is why the story of Chartres is not just the story of a new cathedral. It was at the school of the old cathedral, some 50 years before the church perished in the flames, that theological scholars changed the course of Western thought by introducing a new picture of the cosmos: one that was essential to the emergence of natural science three centuries later. The Gothic arches of Chartres can be considered to be an embodiment of this intellectual breakthrough: it is, in a sense, a temple of science as much as it is the house of God. The 800-year-old building is a testament not only to the faith and ingenuity of the medieval mind but also to the emergence of a new kind of natural philosophy.

That building is the book's central character, and in some ways it could therefore be regarded as a biography - for after all, from the laying of the foundations to the closing of the arches, the time span of Chartres' construction (remarkably short by contemporary standards) is close to a typical human lifetime in the early thirteenth century. By tracing this life story, however, we will discover that, contrary to the common perception, the Middle Ages were not conformist times when nothing changed and when people's lives were ruled by superstition and ignorance. Rather, the flowering of the Gothic style marks a period of genuine originality, an era that can be justifiably regarded as the 'first Renaissance' - and an age that marks the true origins of modern science.

Chartres cathedral was literally born out of miracles. The resolve of the townspeople to construct a new house of God was stiffened by the preservation from the flames of the cathedral's most precious relic: a scrap of cloth said to be from the robe of the Virgin. It was feared at first that this had been consumed in the blaze of 1194; but then the holy relic was found and carried in solemn procession to excite joy and optimism in the towsnfolk. It had been saved by monks who had carried it from the fire and stowed it safely in the crypt. That, at least, is what they said.

The region surrounding Paris - the Île de France - was the nucleus from which Gothic architecture spread across all of northern Europe from the end of the twelfth century. This was the domain of the House of France, which made its peace with the rulers of Chartres-Champagne via a royal marriage in 1160. As it had always resisted the Romanesque tradition that the French kings associated with the foreign authority of Rome, the Île de France had relatively few preconceptions about how a church ought to look. It was the ideal launching ground for a radical departure from tradition, and it hosted all three of the great early Gothic cathedrals, at Chartres, Rheims and Amiens.

Many books on Gothic architecture assume, as they take one through the Royal Portal at Chartres to explore its cavernous shadows, that the reader is an architect. It will not be so here. In providing a brief tour of the building as it stands today, this chapter will not expect you to know your compound pier from your tympanum, your spandrel from your clearstorey. We will encounter these and other elements of medieval church architecture, but more to the point, we will discover how to make some sense of the petrified forest that greets you austerely as you pass across the threshold. It will tell you where to look to find the mason's marks, how to negotiate the tortuous labyrinth depicted on the floor, and how a space like this would have been used and negotiated by monks, priests and worshippers in the twelfth and thirteenth centuries.

'For with much wisdom comes much sorrow; the more knowledge, the more grief.' There was no shortage of warnings, for the biblical scholars of the Middle Ages, about the futility and indeed the peril of knowledge. In the end, the supposedly wise man faces the same fate as the fool, and not all the learning of Solomon (who may have written the words above) would save him from that. The love of God is his only redemption.

But knowledge was worse than useless, because it led unto temptation. That was the dire warning made by St Augustine, the fourth-century philosopher who more or less determined the shape of conventional Christian thinking until the Reformation. Augustine railed against the 'empty longing and curiosity dignified by the names of learning and science', and it was widely held that this curiosity, this need to know, was what led Adam astray. It is no coincidence that, in this climate, St Paul's exhortation noli altum sapere - be not high-minded - was generally rendered and understood as 'seek not to know high things.'

To say that the Christian faith oppressed and hindered science is miusleading, perhaps even meaningless - for just about all proto-scientists in the Middle Ages and beyond were devout Christians. But there was clearly no room for science, as an enquiry into nature, in Augustine's theology.

This chapter examines the intellectual and religious milieu at the beginning of the twelfth century. It traces the tension that developed between the prevailing French ecclesiastical authority of the Cluniac order and its ascetic rival, the Cistercian order of Citeaux, founded around 1100 by St Bernard of Clairvaux. Bernard, as we shall see, has a complex role in the evolution of the Gothic style.

When the medieval person entered a church, they entered heaven. But it is not easy for us now to understand what that meant. When Hollywood wants to show us fantastical or mythical places it reveals its trenchant materialism, and so we have skies and mountains and trees and creatures. They may look like nothing on earth, but Earth is their blueprint. So it is tempting likewise to suppose that the medieval cathedral was intended as a representation of what heaven looks like.

We need only look at medieval paintings to appreciate that this was not so. The schematic, non-naturalistic art of the Middle Ages was not the result of a systematic deficiency in draughtsmanship; artists were simply not trying to mimic reality. Theirs was a world defined by relationships and schema: it was, crudely put, a world of maps, not of photographs. The same is true of the church: it was an image of heaven, but not a theatrical set intended to depict God's realm. Such would, indeed, have risked blasphemy.

In this symbolic language, the cross-shaped plan of the medieval cathedral is not supposed to be an artistic allusion to the crucifixion: the link is more profound than that. Once we understand that Christians of this era considered the physical world itself to be an illusion, a veil that conceals the true realm of God, we can start to see that the builders of cathedrals were attempting to express a transcendent reality through their design.

It may come as a shock, then, to discover how, in this deeply pious age, a church was actually used. It may have been the spiritual heart of the community; but precisely because religion was so central to all aspects of medieval life, townspeople did not necessarily adopt a disposition of hushed awe when they passed through the portals. When a service was not in progress, they would meet their friends in the church, bring their pets for a walk, arrange trysts, eat snacks. The poor might even bed down for the night in the recesses of the church. Stalls clung like limpets to the walls of the building, and wine merchants even sold their wares from the nave of Chartres, until moved to more permanent quarters in the crypt. At Strasbourg, the mayor held office in his pew in the cathedral, meeting burghers there to conduct business.

The services themselves could sometimes be scarcely less riotous. The clergy had no qualms about popularizing their message so that it reached the ordinary person, and to that end a ceremony might be instilled with an element of high theatre, with animals brought in to re-enact the Flight into Egypt or the Adoration of the Magi. The clownish and lewd goings-on during the wintertime Feast of Fools, an echo of the Roman Saturnalia, would scandalize the church today.

It is instructive that the conservative Christians Augustine and Bernard of Clairvaux were beatified, while the medieval innovators are now largely forgotten: men like William of Conches and Thierry of Chartres, who even most scientists today would fail to recognise and acknowledge as their predecessors. We remember one of St Bernard's opponents largely because of a romantic legend that has nothing to do with intellectual debates: Peter Abelard (c.1079-1142) was castrated and banished to the monastery of St Denis for his illicit love of Héloise, a canon's niece.

Abelard was not necessarily the tragic hero - he has been called vain and argumentative, and it has even been suggested that he invented the whole business with Héloise. But he was immensely clever, which is more than can be said for St Bernard (whose greatness 'lay not in the qualities of his intellect, but of his character', as the Encyclopaedia Britannica tactfully puts it). Abelard argued that truth can be found only by asking questions and looking for answers - a fair description, if ever there was one, of the scientific programme. That may sound incontestable now, but to a Christian mystic like St Bernard, insight must come from piety, contemplation and, if you were lucky, divine revelation. While Abelard quoted Virgil approvingly - 'Happy the man who has been able to discern the cause of things' - Bernard considered Abelard a heretic because 'he deems himself able by human reason to comprehend God altogether.'

But St Bernard was not simply fighting a loose-tongued iconoclast; he was trying to resist a rising tide of learning. Europe in the twelfth century was being flooded with Latin translations of the books and manuscripts of the ancient world, derived from Arabic versions by scholars such as Gerard of Cermona and Adelard of Bath (once a student at Chartres, and not to be confused with Abelard). These were mostly texts detailing the scientific knowledge of the Greeks - Euclid, Archimedes, Galen, Ptolemy - along with original contributions from Arabic scholars such as Avicenna and Al-Khwarizmi. This gave rise to a humanistic love of learning equal to anything seen in the later Renaissance; and it was at Chartres that this new knowledge found its most coherent expression. This chapter explores the beginnings of the revival of classical thought in the eleventh and twelfth centuries.

The first universities were religious institutions: the pupils and teachers were monks. They studied at the schools which developed at cathedrals in the twelfth century, and they read more than just the Bible. The academic syllabus of the cathedral school was divided into seven disciplines: the trivium of rhetoric, logic and dialectic, and the quadrivium of arithmetic, music, geometry and astronomy. The conservative 'scholastic' tradition, which flourished in France at Paris, Orléans and Laon, favoured the trivium, which could be (and was) applied in pedantic detail to fine points of scriptural analysis. But at the Chartres school the emphasis fell on the quadrivium: the sciences. It was here that the idea arose that nature is governed by laws and patterns, which we can identify, study and understand.

Curiously, this way of thinking has become attributed to the ancient Greeks, particularly Aristotle. And indeed it is Aristotle who the medieval philosophers selected as their figurehead, particularly once Thomas Aquinas demonstrated in the thirteenth century how Aristotelian thought could be reconciled with Christianity. But the intellectual inferiority complex that afflicted the medieval scholars led them to belittle the originality of their own contribution, so that even now we retain a false picture of Greek philosophy as being predominantly rationalistic rather than recognizing the strongly mystical elements of the ideas of Plato, Pythagorus and those who came after them.

At Chartres, especially under the leadership of Thierry of Chartres (died c.1150), the library began to fill up with the newly translated scientific texts. His brother Bernard, who preceded him as head of the school, gave science its abiding image of how it builds on its predecessors when he acknowledged that 'we are dwarfs on the shoulders of giants, so we perceive more things than they do.'

For Thierry nothing was too sacred that it could not be subjected to the rigours of scientific enquiry: he argued that all rational explanation of the universe depends on mathematics, and he attempted to formulate an interpretation of Genesis in terms of natural causes. This is no indication that Thierry was a cold positivist; in fact he advocated the almost pantheistic ideas of Plato and Pythagorus to justify his belief that there is both an underlying order and an inherent creativity to the universe.

Thierry was assisted in his mission by William of Conches, who was even more bold and outspoken. In the face of vilification from conservative theologians, William threw back at them the accusation that 'because they know not the forces of nature, and in order that they may have comrades in their ignorance, they suffer not that others should search out anything, and would have us believe like rustics and ask no reason.' He mocked the way they would invoke God's mysterious powers to explain everything: ' "We do not know how this is [they say], but we know that God can do it." You poor fools! God can make a cow out of a tree, but has He ever done so? Therefore, show some reason why a thing is so, or cease to hold that it is so.' This was too much: William was called a heretic and forced to resign his post at Chartres.

The stunning architectural invention of Chartres did not, of course, spring up unheralded. When St Bernard of Clairvaux complained in 1124 about 'the measureless height of the houses of prayer, their exaggerated length, their useless width, the amount of stonemasons' work they involve, their paintings which stimulate curiosity and disturb prayer', he was not talking about anything Gothic, but about the excesses (as he perceived them) in the Romanesque churches of the Cluniacs. There was nothing new in the idea that an abbey or a cathedral might be made on a monumental scale.

While Bernard and his fellow Cistercians lambasted Cluniac architectural extravagance, Suger defended the value of costly splendour in a church. He asserted that they enhanced the building's spiritual character by acting as a devotional offering to God. As a close adviser to the French king Louis VI, Suger had an influential voice, and his abbey housed the relics of St Denis, the protector of the French kingdom. So no one was going to prevent Suger from making his new building an imposing construction, or adorning it with gorgeous and elaborate stained-glass windows.

Strange, though, that this new style which sprung from the Île de France should become known by the name of a vanished Germanic tribe. Or perhaps not; for this challenge to the classic Italian style, issuing from the cold lands of northern Europe, would naturally have brought to the mind of Giorgio Vasari, the nationalistic Florentine artist and writer of the sixteenth century, that other, more distant occasion when the barbaric Northern culture burst upon the civilized manners of the Italian peninsula. 'There is also a style which is called the German style', Vasari said. 'Nowadays it is no longer used by men of ability, but is eschewed because it is monstrous and barbaric. It was invented by the Goths after they had destroyed the old Classical buildings and the last Classical architects had perished in the wars of the Volkerwanderung . God preserve all countries from this accursed type of building.'

It was not just Vasari's bad history that persisted for several hundred years, but also its snobbish and xenophobic implication that 'Gothic' implied crude and ugly. Not until the nineteenth century was the Gothic style seen as ingenious and possessed of a creditable aesthetic. By then it was too late to throw away the historical misnomer.

No one planned Chartres cathedral. It's not that we do not know who its architect was; rather, there appears to have been no architect in the sense that we now understand it. When rebuilding of the cathedral began in 1194, there was no budget for it, and no long-term schedule. How could you budget for a building that would take an indefinite time to erect, and which would cost more money than even the church could find in a hurry? This was a project that today would be dismissed as absurd. When there was no more money to pay the workmen, a series of miracles attributed to the Mother of Christ brought donations flooding in.

It's no surprise, then, that some historians have fallen for the romantic myth in which cathedrals like Chartres became community ventures, relying on unpaid labour by devout townspeople rich and poor who sing hymns and chant psalms while wheeling stone blocks and setting them on mortar. That isn't how Chartres was created.

In this chapter I discuss what we know about the former church of Chartres, constructed under Bishop Fulbert around 1020. It was extensively modified subsequently - a fire caused huge damage only a decade or so after its completion - and not only the western façade but also the sanctuary emerged intact enough from the fire of 1194 to be incorporated into the new Gothic building. Indeed, the entire ground plan of the new cathedral was defined by the crypt of Fulbert's edifice, so that the church we see today is, appropriately enough, as much a blend of old and new as a radical departure from tradition.

In the cathedrals of Rheims and Amiens, the names of the 'master builders' responsible for these unimaginably vast projects are cut into the very stone. They wanted future generations to know who they were. But the men who devised Chartres are anonymous, although their work is fit to rival anything by Michelangelo or Brunelleschi.

They were true Renaissance men. They learnt through long, demanding and itinerant apprenticeship how to cut and shape stone - not just into blocks for building, but into the most exquisite garlands and statuary. They knew enough geometry to unfold an entire cathedral from inside a square. They knew not only where they wanted their stones to go, but how to get them there. And they surely needed boundless energy and not a little guile to negotiate with contractors and workmen and suppliers, to reason with impatient and fretful abbots and bishops, and to administer and keep an eye on an ever-uncertain budget.

The astonishing thing is that the master builders in fact had so little tradition on which to draw in the twelfth century. The works of the ancient architects were barely known, available only as sparse hints in encyclopaedic works by Vitruvius and Pliny. Unlike the intellectuals in the cathedral schools, they had no giants' shoulders on which to stand. So they raised giants themselves.    

While the masters of Chartres are a blank page, and those of Rheims and Amiens little more than names, we do have one surviving guide to their character, their knowledge and their aesthetics. The Picard architect and engineer Villard de Honnecourt may have been trained at the Cistercian monastery of Vaucelles at the beginning of the thirteenth century, and he lived to see all of the great cathedrals in the Île de France completed or under construction. He kept a sketchbook of designs, ideas, thoughts and whimsies throughout his career, and although many pages have now been lost, those that remain show how all-consuming was the intellectual curiosity of men like Villard. Alongside drawings of the churches at Chartres, Laon and Rheims, there are pictures of insects, boars, bears, descriptions of lion taming, medical recipes, and many crafty devices to rival anything in Leonardo's better-known notebooks. A water wheel drives and automated saw, while another mechanism explains how to make a saw operate under water. A weight-driven clockwork machine powers an angel who revolves so that his finger is always pointing to the sun. A pivoted brazier in a brass shell acts as a hand-warming device for bishops. And a giant wheel ringed with hinged mallets filled with mercury will turn forever of its own accord - or so Villard claimed.

When they were first discovered, the strange symbols engraved in the stones of Chartres excited much speculation. Were these secret codes of the ancient order of masons, a cipher hiding their long-forgotten mystical knowledge? But as is so often the case, the minds of that earlier age were on more practical matters. Masonry teams were paid according to the amount of work they completed, and so the masons carved their stylized signatures into the stones in order to lay claim to their handiwork.

The mason of the Middles Ages did not labour in some holy rapture, delighted to be building the house of God. He had at least one mouth to feed, and for a project as financially insecure as the construction of a cathedral he needed to be prepared to fight, perhaps even literally, for his wage. So he would take up cudgels against rival teams who threatened to undercut him, and he would go on strike if he was not paid on time. In one account of a church project in twelfth-century France, the workers downed tools when the vegetarian abbot confiscated a pig they had killed for their evening meal.

But they were not always models of virtue themselves. Labourers on York Minster are recorded as stealing materials, arguing with one another, turning up unfit for the strenuous work, or simply not turning up at all. Botched building jobs are no modern affliction. 'For lack of proper care and of roofing there is such a quantity of water that lately a lad has almost been drowned', a report on the York construction complains.

It might also come as no surprise that builders' fees were relatively high. The typical weekly wage of a mason in medieval England - 20 pennies - was three times what he needed to live on, and many of them ran other businesses on the side, renting out carts or tending agricultural plots. For unskilled labourers it was a different story, but the man who knew how to work stone could find a comfortable standard of living.

Statuary at Chartres

This chapter will explore the world of the medieval mason: the working conditions and their attendant hazards, the technical practices, and the structure of this important community which, as the order of freemasons, became the template for the brotherhoods and secret societies that flourished in the Renaissance. We shall see where the stone from Chartres was quarried and how it was transported, and we will discover how the architectural revolution in the Île de France was supported by its fortuitous geology. And we will examine the medieval machinery that made it possible to hoist great blocks aloft and manipulate them into position hundreds of feet above the ground. Construction in the Middle Ages is one of the earliest examples of a mechanical technology that aimed to harness the natural energies of the world.

I shall look also at the breathtaking artistry of these manual workers. The statues of Chartres are every bit as innovative as the architecture itself, and equally characteristic of the Gothic style. They animate the stone, springing to life in a way that the primarily two-dimensional bas-relief of Romanesque sculpture does not. They are a startling marriage of form and function - following the contours of their structural supports (there is no free-standing statuary in early Gothic art) while developing an integrity of their own. These sculptures fill the cathedral with modest exclamations of artistic pride, turning it - if you know where and how to look - into a diverse and absorbing gallery of wonders.

When Frankfurt, Cologne and Ulm were bombed in the Second World War, much of the old cities were reduced to smoking rubble. Yet their Gothic spires never crumbled. The cathedrals may have lost bricks, glass, timber, even walls - but the basic structure of the buildings remained standing. When it was done well, Gothic architecture had a stability that seems almost eternal: the stones lock together in an impervious web of forces.

It would be wrong to suggest that the builders of the great Gothic cathedrals were miraculous engineers with an infallible understanding of mechanics. Sometimes they got it wrong: they tried to raise Beauvais cathedral to such a stupendous height that the pillars and walls could not support it, and it collapsed in 1284. The foundations of the old church on which Chartres cathedral was built were insufficiently solid and had to be partly replaced in the nineteenth century.

Yet these buildings would scarcely have outlived their builders if they had not been put together using sound mechanical principles. The chapter will look at what those principles are, and will examine to what extent they were known to medieval builders.

Arguably, the integration of form with function has never been bettered since the Gothic era. The characteristic pointed arch is not merely an aesthetic choice: it creates less outward pressure at the point where the arch meets the supporting pillars (the capitals) than does the Romanesque rounded arch. This means that pointed-arch vaults are better at transmitting weight downwards, through the capitals, than laterally. As a result, Gothic walls did not need to be so thick and monolithic to avoid collapse, and so they were free to become the 'diaphanous' screens, perforated with light, that are so characteristic of this style.

It was at Chartres that the flying buttress first became an integral element of the design, rather than a post-hoc modification to keep the building standing. These external pillars, connected to the walls by elegant spoked arches, transfer an inward push that counteracts the outward thrust of the walls. They also protect the building against wind pressure, which can be considerable for a structure that rises so awesomely above the plain on which it is constructed.

There are around 1800 images and scenes carved into the stones of Chartres. But most of them are out of view - or would have been to a worshipper of the twelfth century, unequipped with powerful binoculars to spy out high nooks and remote galleries. They were chiselled with great care and sensitivity by a skilled mason, and then carried to some location where the artist could not expect them to be seen again by human eyes.

This apparent perversity tells us everything about the philosophy with which Chartres was constructed, and it could hardly be more distant from 'modern' ideas about the uses and functions of art. Indeed, when even Titian painted an altarpiece three hundred years later, he would have thought as much about his wish to impress the onlooker as his picture's function as an offering to God. But for many of the sculptors of Chartres, God was the only audience they thought they'd ever get, and He was the only one they needed.

In other words, it really did not matter to the creators of Chartres whether any mere mortals saw, appreciated or understood what they had done. The building was a sacred machine, and every part had the primary function of expressing piety and encoding a divine order.

We no longer know how to read this code. It unites the physical with the metaphysical: there is symbolism in every aspect of its design, from the ground plan to the colours of the fabulous stained-glass windows. According to Abbot Suger, building a church involved the 'transposition of the material into the spiritual'.

Artists of later ages, even until the present, have attempted something analogous, but they had no rules. They have struggled to forge materials into an expression of the ineffable; but it becomes a highly personal vision, a reflection of one individual's spiritual world. For all that Kandinsky claimed access to spiritual universals in art, he had no real theory on which to build.

At Chartres it was different. If the cathedral school of Thierry and the philosophy of St Augustine were at odds about the proper boundaries of intellectual enquiry, they were united over one thing: God built the world according to principles of order, which were embodied in number, geometry and mathematics. 'Thou hast ordered all things in measure and number and weight', says King Solomon. It has been claimed that under Thierry, the scholars of Chartres attempted to turn theology into geometry.

That was why temples could not be arbitrarily proportioned, but had instead to reflect and reiterate the divine order of the cosmos. The Bible did not specify the proportions of Solomon's temple in such detail out of sheer pedantry ('The portico at the front of the main hall of the temple extended the width of the temple, that is, twenty cubits, and projected ten cubits from the front of the temple'), but because these dimensions had a holy significance.

At Chartres, this geometric construction was applied to a degree that seems almost obsessive. There is not the smallest feature - the angle of a bevel in a window frame - that was not calculated using some geometrical scheme. It is hard now to separate the spiritual from the practical motivations in all of this, and probably we should not try too hard. For in an age of imperfect record-keeping, lacking even in universal units of measure, construction by geometry turns out to be a splendid way of achieving coherence and consistency.

Thus many of the key proportions at Chartres are simple ratios: 1:2, 1:3, 2:3. These were deemed to be harmonic not just - in fact, not even primarily - in an aesthetic sense, but in a literal sense: they were the ratios of the lengths of strings that produced pleasing harmonies in the tones of a stringed instrument. That was why music was one of the four disciplines of the quadrivium: not because it was a refined and dignified art, but because it held cosmic, divine secrets.

But there are also proportions that might seem at first glance curious and arbitrary: 1:1.414, 1:1.732,1:0.618,. Written another way, however, their geometric origin becomes clear: the first is 1: Ö 2, the second 1: Ö 3. They are the ratios of sides to diagonals in squares or rectangles of simple proportions. The third looks more obscure: 1:( Ö 5-1)/2 - until we recognize it as the Golden Section, the quasi-mystical 'perfect proportion' on which Classical monuments such as the Parthenon were based.

There is a common perception that the great Gothic cathedrals were raised over a century or more, and at Amiens and Rheims that is true. But Chartres was completed by about 1220, only 26 years after it was begun. All the same, it is clearly the work of several master builders: a kind of layer cake in which each horizontal slice bears the distinctive imprint of its creators. These churches would be constructed in a series of 'campaigns', each of them possibly employing a different master, who would bring his own teams and his own ideas about how the stones should be laid. So it is truly remarkable that, with only the sketchiest of drawings, these men succeeded in making a church that retains an overall sense of order and coherence. Only at closer inspection do we see how that unity is underlain by a considerable heterogeneity, and how in fact it is the result of a series of compromises as each master seeks to adapt to the work of his predecessor.

There are, for example, abrupt changes in the quality of the masonry, and we can see areas where the pattern of the stonework changes in mid-span as the walls rise up. By extraordinary good fortune, the very building itself was able to relax and accommodate such variability, because the lime-based mortar could take centuries to fully set. This gave the walls a certain elasticity so that it could adapt to the differential swelling of bricks (which each campaign might take from a different quarry) as they absorb moisture.

It wasn't just the bricks and the workmen that might vary between campaigns, but even the basic principles of measure and geometry. Each master builder would have his own preferred techniques for geometric construction: his own way of dividing up space into regular figures. And there was no standardization of units: several different definitions of the 'foot' were in circulation in the Middle Ages, such as the pied-du-roi (325 mm) and the Roman foot (295 mm). Each master carried his standard unit in the form of an iron square.

Thus cathedrals like Chartres show how great art can spring from many hands, and need not be the work of a single genius. The building is a record of its own making, not simply the embodiment of a pre-existing plan. It is an organic building, full of accidents of growth.

While the design of a cathedral and the heavy labour involved in realising it were generally left to paid specialists, nonetheless it is fair to say that such a project was a communal effort in which everyone had a part to play. Much of the funding was supplied by local donors, and it would be unfair to suppose that they considered they were buying their way into heaven. By financing a cathedral, nobles and wealthy merchants were erecting a buttress that stabilized society. That is why the trades guilds also considered it their duty to contribute, and such gestures have left a secular imprint on the houses of God. The Window of the Trades at Chartres depicts, in glowing reds and blues, no fewer than 43 different professions. A wine seller spurs on the horse pulling his wagon; a shoemaker measures up a pair of disembodied feet. The donors of the Miracles of the Virgin Window are revealed in the bottom panel, where butchers carve up a sheep. The Noah Window was, appropriately enough, funded by the carpenters' guild, whose members are depicted chopping the planks for the Ark.

For a project of this magnitude, the Church was reliant on such contributions despite its great wealth. The diocese of Chartres may have been the largest and richest in all of France in the twelfth century, and its bishop had an annual income equivalent, in a calculation dating from 1956, to a million and a half dollars. Yet the clergy of Chartres were prepared to forego their luxuries when spiritual imperatives demanded it: after the fire of 1194, they agreed to devote all their income for the next three years, beyond the subsistence level, to the rebuilding of the church.

Since the spiritual life of a community could not be suspended while the construction took place, Chartres cathedral was in active use even while it was surrounded by scaffolding and had no roof or windows. It was surely fortunate both for the builders and for the congregation that the age of Gothic building coincided with the Medieval Warm Period, a time of relatively mild climate before the bitterness of the Little Ice Age that Bruegel depicted so memorably. The cathedrals were given temporary wooden roofs, and makeshift services were conducted in the half-built structures. For some people it had to be a matter of faith alone that these great monuments, begun before their birth, would one day stand in all glory long after they were but bones and dust.

'Bright is the noble edifice which is pervaded by the new light', said Abbot Suger. And there was certainly 'new light' in his church at St Denis, where the introduction of elaborate stained-glass windows changed forever the notion of what a church should look like from the inside. In place of a gloom in which people's thoughts were turned inwards toward humility and repentance, there would be a flood of coloured light that inspired awe. By virtue of the new windows in the upper choir of St Denis, Suger proclaimed, the entire church 'would shine with the wonderful and uninterrupted light of most luminous windows, pervading the interior beauty.'

Just as the shape and scale of the stonework in a church could not change so dramatically as it did with the Gothic style without a profound shift in theological and philosophical thinking, so too was the introduction of light and expansive stained glass anchored in a new concept of Christian belief. The spectacular windows of St Denis and Chartres celebrate a metaphysics of light.

This worship of light has mystical roots that stretch far beyond the days of early Christianity. Plato compared light with the notion of good, saying that sunlight is 'not only the author of visibility in all visible things but generation and nourishment and growth.' For his Neoplatonic followers, this elevated light to the status of a transcendental entity, so that when men perceived the truth they were 'illuminated'. 'The divine light penetrates the universe according to its dignity' was how Dante expressed this essentially Neoplatonic idea.

We can see its appeal for Suger. The metaphysics of light was first expounded in a Christian context by an Eastern mystic known as Denis the Pseudo-Areopagite, also called Pseudo-Dionysus, for whom light sustains a universe created by the Divine Light of God. This Denis was conflated, around the ninth century, with St Denis the 'Apostle of France', the patron saint of the kingdom - so that French intellectuals prided themselves in how ancient Greek thought had become transported from Athens to Paris. The abbey of the saint was the traditional burial place of the kings of France, and of tremendous symbolic significance to the monarchy - which indeed is why Suger did not find it hard to secure finances for his rebuilding scheme.

As ever, however, metaphysical schemes cannot be made material without the requisite skills and techniques. Suger could hardly have filled his church with coloured glass if there was not already a technology for making it - stained glass was widely used in earlier French churches, although not to such an extent. The art and science of glassworking in the early twelfth century is described in exquisite detail in the key manual of medieval technology, De diversis artibus , written around 1122 by a Benedictine monk called Theophilus. The book provides a stark reminder of how little the medieval craftsman could take for granted: the section on glassworking commences with a detailed description of how to make a furnace for producing glass. Colouring glass was a trick that owed much to the techniques of ceramic glazing, and its decorative use was allied to the arts of metalwork and jewellery. The blues favoured by Suger were the most expensive forms of stained glass in the Middle Ages, underscoring his belief - which became increasingly prevalent in later centuries - that the value of the materials enhanced the devotional virtue of the work.

Remarkably, some of the blue glass used in French churches embodies in material form the way in which the medieval craftsmen and scholars sought to adopt the traditions and aspirations of the classical world. Theophilus says that the French glass-workers were skilled at recycling blue glass from ancient vessels, which are most probably Roman bottles that once contained exotic unguents. It was also common to transform the opaque glass tiles of Byzantine mosaics into thinner, translucent window glass: literally rebuilding a new culture from the fragments of an older one, and at great expense.

It is well known that history is written by the victors, and there can be little doubt that our standard perception of the trajectories of the arts and sciences is dominated by the scholarly perspective of the fifteenth and sixteenth centuries. By shaping the very language of the debate, this tradition forces us even now to acquiesce to its programme - and so we must talk of the 'Middle Ages', implying a transitional period where little of note happened until the rebirth of learning in the 'Renaissance'.

But in many respects, a true 'humanism' came much earlier than Ficino, Brunelleschi and Alberti. Their revolution was focused largely on the arts and the classics, and it looked down on practical men like Leonardo. The shifts in scientific thinking in the Renaissance were rather more timid than we are often asked to believe, and are layered with a considerable amount of mysticism. The first Renaissance happened in the thirteenth century, and if it retains a stronger theological element than its successor, it is also arguably more profound from the perspective of science, for it marks the beginnings of humankind's enquiry into nature and of our mechanical mastery of matter. This nexus of practical skill, religious innovation and a deep belief in an orderly universe is implied in the fabulous structure that still stands at Chartres.