Monarchs of all they survey

Branwell Bronte studied street-maps of London, as the next best thing to making a real escape from Yorkshire. He held gin-soaked conversations about the city with travellers. They never guessed he hadn't been there, so lovingly had he memorised the geography. But it's a proof of the nature of maps that - of course - he couldn't take himself in. The familiarity you gain from looking at the two-dimensional landscape of symbols cannot be mistaken for the view from any point within it. Map knowledge and direct knowledge can supplement each other, occasionally contradict each other, but never completely overlap. Jorge Luis Borges demonstrated the point by imagining a nation so besotted by cartography that it commissioned a 1:1 map, covering itself from border to border with a life-sized paper translation. Every tree aligned with its charted shadow, but clearly the map never became the country. A symbol is not a thing.

Like this Borges story, philosophical fictions about maps have tended to focus on the paradoxes of knowledge thrown up when the principle behind the Ordnance Survey sheet for Surrey is taken to extremes. In a tradition stretching back to the Enlightenment, they take a commitment to order and reason to the limits where order confutes itself. Borges and Calvino did for maps what Godel's theorem does for the system of numbers, showing that the simple logic of signs implies abysses of strangeness.

You may well think this a desiccated choice of topic. For most of us, the things we represent are much more important than the medium. We want to say stuff with language, read a map in order to travel, use numbers to do sums. But, dry though they may be, such works are celebrations of logic as well as deconstructions of it. They have to be lucid, not just to be pleasurable, but to be intelligible as they demonstrate where lucidity leads.

It's because James Cowan's historical novel is scarcely lucid on any level that A Mapmaker's Dream is a disaster. Packaged as small-format mind-food on the model of Dava Sobel's Longitude, this story of a Venetian mapmaker who assimilates the world without leaving his monastic cell is remorselessly fuzzy. To tell their tales to Fra Mauro so he can fix new wonders on his map come a ship's captain, a learned Jew, a Levantine merchant. They spout awkward verbiage: "Good friar, my monastery happens to be the world." They report unprodigious prodigies (a statue oozing honey, people who worship the wind). In return, Fra Mauro's commentary flubs distinctions, softens differences, draws vacuous conclusions. "Had they not enjoyed during their lifetime an uncommon clarity, and perception, in their worship of nature, which had granted them the grace of such freedom? It was hard to tell." It is hard to tell anything, from sentences where you can shuffle all the abstract nouns without making less (or more) sense. Though the irony of Fra Mauro's stationary globe-trotting is supposed to be important, map knowledge and the knowledge of the senses go indifferently into Cowan's Magimix.

Theoretically, the whole of Asia as the Renaissance imagined it lies at his disposal - the Other to Europe's Self. But that way of figuring the line between known and unknown doesn't interest Cowan, either. The book is vague in the service of a vision which turns everything to Self in the end. So travellers "are forever trudging towards the prospect of knowing more about themselves". Christian missionaries are "more intent on transforming themselves". For a moment, when a papal emissary describes Tartars listening to the sunrise and drinking fermented mares' milk, or "cosmos", it looks as if difference stands a chance. But no: "Whether it is cosmos we drink in or the sound of the sun, each draught leaves us feeling more intimately at one with ourselves." Cowan may think he has given Fra Mauro a tolerant gaze. In fact, the insistence that everything is in inward agreement homogenises the world. Whatever Fra M sees, turns into Fra M. The outlook of A Mapmaker's Dream belongs to the New Age rather than the Age of Reason.

In any case, do maps have much to do with dreams - real, sleeping dreams whose currents tell us what we want even if we don't want to want it? The knowledge maps represent seems much more conscious. It's daydreams that they reflect, as Jeremy Black's fascinating study of the historical atlas makes clear. Maps of the past are moulded directly by the present's favourite ideas. The tabling of history on folded paper began because the Bible and the Classics gave Europeans an imaginative stake in Greece and Palestine. The invention of chromolithography in the 1850s gave British cartographers the opportunity to extend Johannes Hubner's original idea of coloured countries by filling the globe with imperial pink. Nazi mapmakers loved arrows. Wicked, menacing arrows showed foreigners attacking Germany; dynamic, virtuous arrows showed the Master Race hitting back. These were the graphic expressions of the waking mind's schemes. Maps aren't dreams. "Last night I dreamed I went to Manderley again". OK, but you didn't dream it had 8.3 hectares of deciduous woodland crossed by an unimproved B-road, did you?