BOOK REVIEW / Here be no dragons: 'New Worlds, Ancient Texts' - Anthony Grafton: Belknap Harvard, 23.95 pounds

THE PLOT of a complicated and, it has to be said, quite implausible detective story by Dorothy Sayers turns on the fact that Lord Peter Wimsey, who knows everything, is familiar with a book called Cosmographia, published in Basel in 1550 by one Sebastian Munster.

A man of prodigious learning - he was, for example, though not a Jew, one of the great Hebrew scholars of his day - Munster was also a man in a dreadful muddle. His maps, for example, show the New World where it is, while his text mixed it up with the Indies. He took account of the latest geographical discoveries; yet he served them up with maps that said 'Here be dragons', like the most superstitious products of the Middle Ages. And he spiced them with a sauce of the silly old stories from ancient authors that Othello told so well, of 'men whose heads grow downward from their shoulders', not to mention men with one big foot they used as a parasol in hot weather.

Sebastian Munster shared his muddle with greater men than himself: with the great astronomer Copernicus, for example, and with the brilliant anatomist Vesalius. Copernicus put the sun, not the earth, in the middle of his system; but he still imagined the planets turning in solid, crystalline spheres. Vesalius repeated the experiments of his greatest predecessor in the ancient world, Galen, and found that blood could not pass through the heart from auricles to ventricles. But in his first edition Vesalius insisted on what the books told him, even though his own eyes told him it was untrue.

Munster, Vesalius and Copernicus, in other words, were caught in the dilemma of the Renaissance, halfway out from one bank to the other in the boldest intellectual river-crossing the European mind has attempted. Anthony Grafton's book is a study for the general reader of that crossing from the medieval world where the authority of books, sacred and classical, reigned unchallenged, to the modern world with its preference for empirical evidence.

It was sailors, not scholars, who actually discovered new worlds. In the very year when Munster's learned maps were decorated with fabulous monsters, portolan charts were available along Mediterranean waterfronts that looked like modern maps, for good reason: they were based on the experience of men who sailed those coasts, and who would drown if they got them wrong.

In the 40 years after 1572, Grafton says, 'the balance of scientific authority had gradually shifted'. The discovery of new planets and comets destroyed the ancient belief that the heavens were unchanging, and the discovery that blood passed from one side of the heart to the other, not through the septum, but through veins, arteries and capillaries, were only two of the most striking illustrations of the general point: by the early 17th century most scholars no longer believed that the ancients knew as much as the moderns.

Grafton lays some stress in his description of this process on the part played by the New World in the literal sense: by the discovery of the Americas. He includes substantial sections on the accounts of the Meso-American Indians by the explorers, the Conquistadors and their critics, such as Bartolome de Las Casas; on tobacco and the other newly discovered herbs; and on syphilis.

That is natural in a book conceived as a commemoration of Columbus's voyage. Grafton was originally invited to organise an exhibition of the New York Public Library's holdings of books on 16th- and 17th-century European thought, and to write a book about the effect of the voyages on European thought. This is that book.

The direct effect of the voyages was undoubtedly considerable. Grafton cites a good illustration. The Jesuit father, Jose de Acosta, was surprised to find that it could be so cold in March in the tropics that he had to go inside to get warm. 'What could I do then but laugh at Aristotle's Meteorology and his philosophy?'

The fact is, however, that the transformation of European thought between 1450 and 1700 was only peripherally affected by the discovery of America, tremendous as the consequences of that event certainly were. The greatest voyage of discovery took place between the ears of daring and intelligent Europeans.