The difference between animals and human beings is that the later claims the capacity to record cruelty.

The ability to write and to think is ostensibly a differentiating factor but in reality it is meaningless because people do not think. They merely use existing prejudices to justify every cruelty and that makes man the most cruel and destructive animal in the world.

In the summer of 1580, Montaigne acted on the desire of a lifetime, and made his first journey outside France, setting off on horseback to Rome via Germany, Austria and Switzerland. He travelled in the company of four young noblemen, including his brother, Bertrand de Mattecoulon, and a dozen servants. They were to be away from home for seventeen months, covering 3,000 miles. Among other towns, the party rode through Basle, Baden, Schanhausen, Augsburg, Innsbruck, Verona, Venice, Padua, Bologna, Florence and Siena - finally reaching Rome towards evening on the last day of November 1580.

As the party travelled, Montaigne observed how people's ideas of what was normal altered sharply from province to province. In inns in the Swiss cantons, they thought it normal that beds should be raised high off the ground, so that one needed steps to climb into them, that there should be pretty curtains around them and that travellers should have rooms to themselves. A few miles away, in Germany, it was thought normal that beds should be low on the ground, have no curtains around them and that travellers should sleep four to a room. Innkeepers there offered feather quilts rather than the sheets one found in French inns. In Basle, people didn't mix water with their wine and had six or seven courses for dinner, and in Baden they ate only fish on Wednesdays. The smallest Swiss village was guarded by at least two policemen; the Germans rang their bells every quarter of an hour, in certain towns, every minute. In Lindau, they served soup made of quinces, the meat dish came before the soup, and the bread was made with fennel.

French travellers were prone to be very upset by the differences. In hotels, they kept away from sideboards with strange foods, requesting the normal dishes they knew from home. They tried not to talk to anyone who had made the error of not speaking their language, and picked gingerly at the fennel bread. Montaigne watched them from his table:

Once out of their villages, they feel like fish out of water. Wherever they go they cling to their ways and curse foreign ones. If they come across a fellow-countryman . . . they celebrate the event .. . With a morose and taciturn prudence they travel about wrapped up in their cloaks and protecting themselves from the contagion of an unknown clime.

In the middle of the fifteenth century, in the southern German states, a new method of heating homes had been developed: the Kastenofen, a freestanding box-shaped iron stove made up of rectangular plates bolted together, in which coal or wood could be burnt. In the long winters, the advantages were great. Closed stoves could dispense four times the heat of an open fire, yet demanded less fuel and no chimney-sweeps. The heat was absorbed by the casing and spread slowly and evenly through the air. Poles were fixed around the stoves for airing and drying laundry, and families could use their stoves as seating areas throughout the winter.

But the French were not impressed. They found open fires cheaper to build; they accused German stoves of not providing a source of light and of withdrawing too much moisture from the air, lending an oppressive feeling to a room.

The subject was a matter of regional incomprehension. In Augsburg in October 1580, Montaigne met a German who delivered a lengthy critique of the way French people heated their houses with open fires, and who then went on to adumbrate the advantages of the iron stove. On hearing that Montaigne would be spending only a few days in the town (he had arrived on the 15th and was to leave on the 19th), he expressed pity for him, citing among the chief inconveniences of leaving Augsburg the 'heavy- headedness' he would suffer on returning to open fires - the very same 'heavy-headedness' which the French had long condemned iron stoves for provoking.

Montaigne examined the issue at close quarters. In Baden, he was assigned a room with an iron stove, and once he had grown used to a certain smell it released, spent a comfortable night. He noted that the stove enabled him to dress without putting on a furred gown, and months later, on a cold night in Italy, expressed regret at the absence of stoves in his inn.

On his return Home, he weighed up the respective qualities of each heating system:

It is true that the stoves give out an oppressive heat and that the materials of which they are built produce a smell when hot which causes headaches in those who are not used to them ... On the other hand, since the heat they give out is even, constant and spread all over, without the visible flame, smoke and the draught produced by our chimneys, it has plenty of grounds for standing comparison with ours.

So what annoyed Montaigne were the firm, unexamined convictions of both the Augsburg gentleman and the French that their own system of heating was superior. Had Montaigne returned from Germany and installed in his library an iron stove from Augsburg, his countrymen would have greeted the object with the suspicion they accorded anything new;

Each nation has many customs and practices which are not only unknown to another nation but barbarous and a cause of wonder.

When there was of course nothing barbarous nor wondrous about either a stove or a fireplace. The definition of normality proposed by any given society seems to capture only a fraction of what is in fact reasonable, unfairly condemning vast areas of experience to an alien status. By pointing out to the man from Augsburg and his Gascon neighbours that an iron stove and an open fireplace had a legitimate place in the vast realm of acceptable heating systems, Montaigne was attempting to broaden his readers provincial conception of the normal - and following in the footsteps of his favourite philosopher:

When they asked Socrates where he came from, he did not say 'From Athens', but 'From the world.'

This world had recently revealed itself to be far more peculiar than anyone in Europe had ever expected. On Friday 12 October 1492, forty-one years before Montaigne's birth, Christopher Columbus reached one of the islands on the archipelago of the Bahamas at the entrance of the Gulf of Florida, and made contact with some Guanahani Indians, who had never heard of Jesus and walked about without any clothes on.

Montaigne took an avid interest. In the round library were several books on the life of the Indian tribes of America, among them Francisco Lopez de Gomara's L'histoire generate des Indes, Girolamo Benzoni's Historia de mondo novo and Jean de Lery's Le voyage au Bresil. He read that in South America, people liked to eat spiders, grasshoppers, ants, lizards and bats: they cook them and serve them up in various sauces. There were American tribes in which virgins openly displayed their private parts, brides had orgies on their wedding day, men were allowed to marry each other, and the dead were boiled, pounded into a gruel, mixed with wine and drunk by their relatives at spirited parties. There were countries in which women stood up to pee and men squatted down, in which men let their hair grow on the front of their body, but shaved their back. There were countries in which men were circumcised, while in others, they had a horror of the tip of the penis ever seeing the light of day and so 'scrupulously stretched the foreskin right over it and tied it together with little cords'. There were nations in which you greeted people by turning your back to them, in which when the king spat, the court favourite held out a hand, and when he dis charged his bowels, attendants 'gathered up his feces in a linen cloth'. Every country seemed to have a different conception of beauty:

In Peru, big ears are beautiful: they stretch them as far as they can, artificially. A man still alive today says that he saw in the East a country where this custom of stretching ears and loading them with jewels is held in such esteem that he was often able to thrust his arm, clothes and all, through the holes women pierced in their lobes. Elsewhere there are whole nations which carefully blacken their teeth and loathe seeing white ones. Elsewhere they dye them red. The women of Mexico count low foreheads as a sign of beauty, so while they pluck the hair from the rest of their body, there they encourage it to grow thick and propagate it artificially. They hold large breasts in such high esteem that they affect giving suck to their children over their shoulders.

From Jean de Lery, Montaigne learned that the Tupi tribes of Brazil walked around in Edenic nudity, and showed no trace of shame. Indeed, when Europeans tried to offer the Tupi women clothes, they giggled and turned them down, puzzled why anyone would burden themselves with anything so uncomfortable.

De Lery's engraver, who had spent eight years with the tribes, took care to correct the rumour rife in Europe that the Tupis were as hairy as animals. The men shaved their heads, and the women grew their hair long, and tied it together with pretty red braids. The Tupi Indians loved to wash; any time they saw a river, they would jump into it and rub each other down. They might wash as many as twelve times a day.

They lived in long barn-like structures which slept 200 people. Their beds were woven from cotton and slung between pillars like hammocks. When they went hunting, the Tupis took their beds with them, and had afternoon naps suspended between trees. Every six months, a village would move to a new location, because the inhabitants felt a change of scene would do them good. The Tupis' existence was so well ordered, they frequently lived to be a hundred and never had white or grey hair in old age. They were also extremely hospitable. When a newcomer arrived in a village, the women would cover their faces, start crying, and exclaim, 'How are you? You've taken such trouble to come and visit us!' Visitors would immediately be offered the favourite Tupi drink, made from the root of a plant and coloured like claret, which tasted sharp but was good for the stomach.

Tupi men were allowed to take more than one wife, and were said to be devoted to them all. Their entire system of ethics contains only the same two articles: resoluteness in battle and love of their wives,' reported Montaigne. And the wives were apparently happy with the arrangement, showing no jealousy. Sexual relations were relaxed, the only prohibition being that one should never sleep with close relatives. Montaigne, with his wife downstairs in the castle, relished the detail:

One beautiful characteristic of their marriages is worth noting: just as our wives are zealous in thwarting our love and tenderness for other women, theirs are equally zealous in obtaining them for them. Being more concerned for their husband's reputation than for anything else, they take care and trouble to have as many fellow wives as possible, since that is a testimony to their husband's valour. It was all undeniably peculiar. Montaigne did not find any of it abnormal.

He was in a minority. Soon after Columbus's discovery, Spanish and Portuguese colonists arrived from Europe to exploit the new lands and decided that the natives were little better than animals. The Catholic knight Villegagnon spoke of them as 'beasts with a human face'. The Calvinist minister Richer argued they had no moral sense and the doctor Laurent Joubert, after examining five Brazilian women, asserted that they had no periods and therefore categorically did not belong to the human race.

Having stripped them of their humanity, the Spanish began to slaughter them like animals. By 1534, forty-two years after Columbus's arrival, the Aztec and Inca empires had been destroyed, and their peoples enslaved or murdered. Montaigne read of the barbarism in Bartolomeo Las Casas's Brevissima Relacion de la Destruccion de las Indias, printed in Seville in 1552, translated into French in 1580 by Jacques de Miggrode as Tyrannies et cmautes des Espagnols perpetrees es Indes occidentales qu'on dit Ie Nouveau Monde. The Indians were undermined by their own hospitality and by the weakness of their arms. They opened their villages and cities to the Spanish, to find their guests turning on them when they were least prepared. Their primitive weapons were no match for Spanish cannons and swords, and the conquistadores showed no mercy towards their victims. They killed children, slit open the bellies of pregnant women, gouged out eyes, roasted whole families alive and set fire to villages in the night.

They trained dogs to go into the jungles where the Indians had fled and to tear them to pieces.

Men were sent to work in gold and silver mines, chained together by iron collars. When a man died, his body was cut from the chain, while his companions on either side continued working. Most Indians did not last more than three weeks in the mines. Women were raped and disfigured in front of their husbands.

The favoured form of mutilation was to slice chins and noses. Las Casas told how one woman, seeing the Spanish armies advancing with their dogs, hanged herself with her child. A soldier arrived, cut the child in two with his sword, gave one half to his dogs, then asked a friar to administer last rites so that the infant would be assured a place in Christ's heaven.

With men and women separated from each other, desolate and anxious, the Indians committed suicide in large numbers. Between Montaigne's birth in 1533 and the publication of the third book of his Essays in 1588, the native population of the New World is estimated to have dropped from 80 to 10 million inhabitants.

The Spanish had butchered the Indians with a clean conscience because they were confident that they knew what a normal human being was. Their reason told them it was someone who wore breeches, had one wife, didn't eat spiders and slept in a bed:

We could understand nothing of their language; their manners and even their features and clothing were far different from ours. Which of us did not take them for brutes and savages? Which of us did not attribute their silence to dullness and brutish ignorance? After all, they were unaware of our hand-kissings and our low and complex bows.

They might have seemed like human beings: 'Ah! But they wear no breeches . . .'

Behind the butchery lay messy reasoning, and it will never cease until we cross the borders of prejudice.