In the western world in the 20th century we've thrown off our clothes to adore the body, but in the Sahara they still wrap up. I encountered a taboo among Tuareg men - the nomads of the central Sahara - about showing their mouths to strangers. They wear a turban around the head and across the nose and mouth, only loosening it among friends. If a stranger arrives, there's a sudden flurry of turban-wrapping. Even while eating they keep their mouths covered, passing handfuls of food under the front of turbans which bulge rhythmically as they chew.
The beauty of the desert lies not in concealment but in revelation. It is unvegetated, naked, a place of big geographic statements: Sand, rock, sky.
Over the past couple of years I have travelled to most of the world's deserts to write a book about them. I took this photograph (right) in Namibia at the dunes of Sossusvlei, the loveliest dunes I have seen. You set out before dawn to reach Sossusvlei, as the park rangers open the gate at five. There follows an hour-long race on a bone-rattling track, with several drivers vying to be first and avoid inhaling everyone else's dust. The road runs due east through the Namib Desert, towards the sea. As the sun rises you realise you are driving through a canyon; slowly its walls turn to copper, the yellow sand of the Namib burnished by wind-blown red sand from the distant Kalahari.
The sun is rising by the time you reach the water hole at Sossusvlei. Several paths lead to the tops of nearby dunes, but if you are feeling more adventurous climb farther, and higher. You can find yourself on the top of what are said to be the highest sand dunes on earth. The view, 20 miles out across sand, stretching like the Himalayas, is astounding. The dunes, made by winds that gust from every direction, are star-shaped and sinuous.
To my eye, the beauty of the desert arises from the absence of information. The modern world is cluttered with messages, often commercial, mostly inessential. Imagine the contrast with a Saharan landscape where you see no advertisements, not even a Coca-Cola sign, for several hundred miles.
There are other, equally natural environments. But the desert's virtue is monotony - it is a meditation on a single note. Up against the clean desert, without external stimulus, you find the stuff of your mind bubbling to the surface. Some people find it makes them see themselves: a revelation. Jesus went into the arid Judean wilderness for 40 days and nights, and scores of Christian mystics have followed him there.
A 20th-century mystic who loved the desert was Aldous Huxley. At Llano, in California's Mojave Desert, I stood among the sagebrush, beside the wooden house where he lived in the Forties. A few miles off was the dried lake bed of the Edwards airforce base, where the Space Shuttles land, and a huge blue sky barrelled overhead. The present occupant of the house, a screenplay writer called Richard Linnett, looked at me with a grin.
"Huxley learned to drive out here," he said.
We both began to laugh.
"Can you imagine it?" asked Richard.
At the age of 17, an illness had left Huxley almost blind. But in his forties he studied the techniques of the alternative therapist WH Bates, and improved his eyesight appreciably. He found that the brilliant desert light made it easier for him to see - hence the surreal image of the purblind prophet bowling across the dusty Mojave in his beat-up Chevy truck.
Bates believed that some exposure of the eyes to direct light was beneficial - so we may be wrong to hide our eyes from the sun, spending millions on the latest shades. When I travelled through the Sahara, my guide,Ya-ya, contemptuously turned down my offer of sunglasses when he was driving. A few weeks later, in any case, sand had turned their lenses into milky blurs, so I threw them away.
But you do need some eye protection in the desert. As we climbed one dune at dusk a breeze began to blow, plucking the crests into mists of sand. A gale from the north soon turned the desert into a choppy sea. That night we sat around the fire turbanned and swaddled. There was sand in the food, sand in our mouths and sand in our eyes.
To fly direct to Windhoek, Namibia, from Heathrow, call Air Namibia (0181 944 6181).