Just think of the greatest adventurers who ever lived and the greatest journeys ever undertaken: the Jews, Marco Polo, Christopher Columbus and Charles Darwin come to mind. All of them had compelling reasons for setting off on dangerous journeys into the unknown. What they found (in their cases the Promised Land, China, America and evolution respectively) soldered them into history and made them famous, but also opened the world to travel as never before.

Travel writing ever since has echoed the odysseys of these great people. Writers still feel it incumbent on them to have some higher purpose to their journeys beyond mere self-indulgence or curiosity. On the rare occasions when travel writers break this rule they tend to fall ill or become irredeemably cranky when they sit down to put their experiences on paper.

The range of reasons travel writers dream up to focus their journeys range from the absurd to the sublime. Take that outstanding wordsmith Bill Bryson. This man literally thought up journeys he could take, to create fodder for his witty irony and superb humorous descriptions. A walk along the Appalachian Trail with an old school friend (do you remember Katz?) became much more than 'A Walk in the Woods' as it was entitled. It was a humorous ramble through the American nature tourist culture and a lambasting of the authorities responsible for the national parks of the United States. It did not matter that Bryson completed only a tiny part of the trail. This incredibly long hike (Bryson spends a few pages embarrassing all the authorities who cannot agree on its exact length) served one purpose and one purpose only; it gave Bryson something to write about.

Similarly Bryson's book about rural America entitled 'The Lost Continent' has a very thin basis to it: Bryson vaguely travels the roads his parents followed, when they took their children on madcap long haul treks across the United States to see the sights (and sites of famous battles and historical occurrences) and generally scrounged their way along on a shoestring budget, to the mystification of the Bryson children. Again Bryson gets his teeth into a subject without much justification. Not that he needs it, you understand.

Bryson made a career of taking whole continents and wrapping them around his tongue, as in 'Down Under', his dry yet informative take on Australia. He went there because he had always wanted to see it and, as the subtext suggests, he was looking for an alternative place to live. He and his family had already done England and New England. As it happened, the Bryson family returned from New Hampshire to Britain, giving down under the thumbs down. Just too many snakes per square kilometer I suppose.

Now we come to the sublime reasons for travel. There are tales of pilgrimage, such as Shirley MacLaine's account of her walk the length of the Santiago de Compostela Camino in northern Spain, the ancient 500 mile pilgrimage route initiated by St James de Compostela ending at Santiago. 'Camino: a journey of the spirit' never reaches any conclusions and elicits no discernible greatness of spirit in the writer, but it surely gave Ms MacLaine fodder for a bestselling book in the bland genre of Californian spiritualism.

Ineffably more substantial is the marvelous book by William Dalrymple 'From the Holy Mountain' in which this handsome young Scot journeys to the places visited by John Moschos some 1500 hundred years before. His beautiful journey through the dying remnants of Byzantium in our own age (he traveled in 1997) is an unforgettable book by a marvelously intelligent Catholic probing the embers of Eastern Orthodox religion.

Between the absurd and the sublime reasons for travel lie many others. In 'African Rainbow' Lorenzo and Mirella Ricciardi traveled along the waterways in Africa, evidently searching for the ultimate noble savage in the European mold. They never found him or her but their book was published. It ends up being an uneasy journey of a couple to a continent they didn't understand.

In 'The Great Railway Bazaar' Paul Theroux travels on the Orient Express, the Khyber Pass Local, the Golden Arrow, the Mandalay Express, an odyssey on great trains from London through Europe and Asia, across Siberia. And his eye misses nothing as he describes this travel mode of a bygone age and these out-of-the-way places, but I always feel that Theroux travels and writes under duress rather than from compulsion, rather like Shiva Naipaul in 'North of South'.

Naipaul visited the insalubrious African countries: Zambia, Tanzania and Kenya, where Asians have been personae non grata in the past, and in some places still are, to find out what makes Africa tick. Of course no one does know what makes Africa tick, not even Naipaul.

Never mind that these men seem to have been uncomfortable about their journeys. Both are renowned travel writers, not least due to their dogged purposefulness. The point, it seems, is to have some intention when moving across the landscape. A traveler without intention is merely a wanderer.