ART / The page of reason: Cassiano dal Pozzo, Galileo's publisher, Poussin's patron, built a museum made of paper. Andrew Graham-Dixon reports

'THE Paper Museum of Cassiano dal Pozzo', a small part of which is currently on exhibition in the British Museum's department of prints and drawings, brings a dead world to life with sudden and unexpected vividness. This is an unusual museum: not a real one, but a notional one; the idea of a museum, one of the earliest ancestors of Andre Malraux's musee imaginaire. It exists only on paper. It is a museum that takes the form of upwards of 1,500 drawings and watercolours and that contains, in reproduction, each shrunk to the manageable dimensions of an image on a single sheet of paper, an apparent infinitude of weird bits and pieces: it is full of fragments, odds and ends, peculiar visual bric-a-brac ranging from a drawing of The Sarcophagus of 'Alexander Severus' to a drawing of the largest broccoli plant ever grown. But it adds up to more than the sum of its parts and its chief exhibit might be said to be itself. Much of it is devoted to recording the relics of the ancient world, but it is most fascinating as a relic of its own time, and of a culture in which the curious idea of a paper museum could have taken root and could have grown such strange fruit.

Cassiano dal Pozzo, was once generally accepted to be one of the most extraordinary men of his time but these days he requires introduction. Born in Turin in 1588, he was brought up in Pisa and spent his adult life in Rome, where under the patronage of Pope Urban VIII he was appointed to a position in the household of the pontiff's nephew, Cardinal Francesco Barberini. His official activities seem to have been fairly undemanding, if the range of his unofficial activities is anything to go by. Cassiano is known to have commissioned some 40 paintings by Poussin, including the great Seven Sacraments. He was also an archaeologist, a botanist, a zoologist, a geologist and the leading ornithologist of his age. He played a vital role in the publication of Galileo's The Assayer of 1623, a book that set itself the small task of changing the accepted picture of the known universe by finally demolishing the old Ptolemaic cosmology. Without Cassiano, the idea that the earth revolves around the sun, rather than the other way around, might have caught on rather later than it did.

Cassiano was also a driving force behind the Accademia dei Lincei, a scientific academy that was one of the least characteristic but most important institutions of Counter-Reformation Rome - in the days of the Inquisition, scientists tended to be thought of as a good way to start a bonfire - and as such he played an important part in both aspects of the 17th century's scientific revolution. He contributed to the dismantling of old Aristotelian approaches to the study of flora and fauna and the forging of a new, empirically based natural history. If they had played football in 17th-century Italy he probably would have been player-manager of Juventus.

There is something more than slightly daunting, to the modern mind, about the sheer diversity of Cassiano's interests and talents. To consider his career is to realise with renewed force that we live in an age of extreme specialisation and distinctly partial individual competence. How many ornithologists, these days, are also expert in art history? (And how many art historians are familiar with the migratory behaviour of the citron-crested cockatoo?) To Cassiano, the notion that aesthetic and scientific inquiry represented two different forms of endeavour would have seemed alien, since in his day the arts and the sciences went under the same name, both being referred to as 'art'.

Visitors to the renovated 16th-century palace where Cassiano lived in Rome would have passed through the courtyard where the live birds that he kept for the purposes of study could be seen perching on his collection of classical sculptures; inside, they would have passed the laboratory which he had had constructed to carry out animal dissections and, proceeding through corridors lined with maps, they would have met their host upstairs, on the piano nobile, surrounded by paintings on sacred and pagan themes by Poussin. Cassiano's house was a microcosm of his interests and, also, the image of Western civilisation on the verge of enormous change, being transformed by new knowledge and new approaches to the getting of knowledge. The house cannot be revisited, but one of the things that it contained can: Cassiano's Paper Museum, which in its own way is also a model of his mind and of the 17th century's fast-turning world.

The origins of the Paper Museum seem to have lain in a desire to make sense of what, to a cultured 17th-century Italian, would have represented both heritage and competition: the vast but shattered legacy of the classical world. Cassiano conceived his Paper Museum as a kind of pictorial encyclopaedia that would one day (he hoped) contain a record of the appearance of every single existing trace of the great Greek and Roman civilisations of the distant past. He commissioned those whom he considered to be the most promising artists of the day (and showed fairly good judgement: they included, besides Poussin, Pietra Testa and Pietro da Cortona) to make copies of every such relic that he knew of. The Paper Museum found room for everything: for the major works of classical antiquity, monumental marble friezes, grand reliefs sketched in minute detail by Cassiano's army of copyists; but also for all kinds of other arcana. It contained images of antique brooches and weighing and measuring devices, of Roman boxing gloves, of plates and dishes and jugs and of all kinds of votive objects including at least one large stone phallus.

Looking, today, at the drawings that must have been delivered almost daily to the Dal Pozzo household for cataloguing and entry into the Paper Museum, what is most striking about them - and this is all the more striking, given that the artists responsible for them included some of the great masters of 17th-century painting - is their peculiar artlessness. They are not just pictures of objects, but pictures of an attitude to those objects. The drawings almost invariably attempt to include the maximum of visual information about the things drawn. The light is uniform and even, the viewpoint chosen absolutely straightforward; no attempt is made - and this is where these drawings of the remains of classical antiquity differ so radically from those of later centuries, particularly the 18th and 19th - to impart mood or drama through composition or effects of chiaroscuro.

A Neoclassical artist, drawing the Marble Statue of a Man Wearing a Toga which Pietro da Cortona drew for Cassiano sometime in the 1620s or 1630s, would almost certainly have attempted to make something of the pathos of the figure's headless state, would have dwelt on the deep carving of the drapery and would have modelled it in much greater tonal contrast. But Pietro da Cortona's two drawings have the matter-of-factness of a fashion plate: this, they say, is how Romans might have been wearing their togas circa AD 100. They suggest a culture fascinated by the minutiae of the past: one that saw the relics of Roman civilisation not primarily as great works of art but also as vital keys to an understanding of its social habits and customs. The Paper Museum is full of drawings - Vincenzo Leonardi's study of a Samnite Triple Breast-Plate is another beautiful, eagle-eyed example - that take more than one view of a single object and whose aim appears to be to impart something like the knowledge that might be derived from handling a thing. These images are attempts to piece the past together, to reconstruct it in order to understand it. The only artful, overtly imaginative drawings of classical subject matter in the Paper Museum take the form of reconstructions: a Roman meal, imaginary diners gathered round reclining, reconstructed from the evidence of classical literature or a theatrical performance, reconstructed from the evidence of a crumbling, ruined amphitheatre.

Cassiano dal Pozzo's Paper Museum sheds much light on the paintings of Poussin (there are worse ways to spend a morning in London at the moment than by following a visit to this show with a trip to the National Gallery to look at the Poussins), explaining among other things what can, occasionally, seem that artist's excessive devotion to archaeological fidelity. The Paper Museum also suggests, like Poussin's paintings of bacchanalia poised between order and discord, a world attempting but not entirely succeeding in squaring its knowledge of antique civilisation and customs with its own sense of civilisation and of what constitutes acceptable behaviour: a world potentially troubled as well as inspired by antiquity, by its violence and pagan religious practices. Cassiano's encyclopaedic attempt to gather together all the visual evidence of antiquity was bound to turn up evidence not only of the great intellectual and architectural and organisational achievements of Roman civilisation, but also of its savagery: the Paper Museum contains drawings of gladiators being mauled by wild beasts, of arcane religious rites and strange obscure objects of worship.

Cassiano lived at a time when knowledge must have seemed particularly fraught with worrying possibilities. The letters of his contemporaries suggest the frequent doubts and anxieties that coexisted, in them, with an insatiable thirst for knowledge: Galileo's discoveries did not merely redefine the world, they also turned it upside down.

The Accademia dei Lincei, the Academy of Lynxes, named itself knowingly after that small and extremely sharp-eyed animal, and the natural history drawings assembled in the Paper Museum testify both to the lynx-eyed nature of Cassiano and to his determination to forge a new method of scientific inquiry based on close observation. The drawings of fruit and birds and stones and virtually all the known phenomena of the natural world in the Paper Museum suggest that Cassiano wanted to understand and tabulate nature just as exhaustively as he wanted to reconstruct and comprehend classical antiquity. Cassiano lived in the same period that saw the invention of both the microscope and the telescope, and he commissioned the first ever microscopic drawings of natural objects. The natural history drawings in the Paper Museum - fantastic drawings in close focus of oranges, tomatoes, herons, quartz stones - are utterly remarkable, because the nature of their realism sets them apart from virtually any other images made before them (although they have something of Leonardo's curiosity about them: Leonardo understandably fascinated Cassiano). This is not the moralised realism of Dutch still-life painting, objects caressed and renounced at the same time, but the neutral realism of analysis: the kind of precise observation that Cassiano encouraged in his draughtsmen would, eventually, make the taxonomic rigour of Linnaeus possible.

There is even, in a few microscopically realised drawings of ammonites, a sense that Darwin is there, waiting in the wings: such apparent freaks of nature fascinated Cassiano because they did not fit any known explanation of the origins of life, because they simply did not tally with biblical accounts of the creation. To visit the Paper Museum is, above all, to see the world being seen differently. Through the eyes of science, perhaps; or as Cassiano dal Pozzo would have said, 'through the eyes of the lynx'.

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