She falls silent. 'Does that answer your question?' asks the dean. The questioner gives a huge, expressive shrug and sits down.

There is still the dinner to get through. In half an hour the president is to host a dinner at the Faculty Club. Initially he and Norma had not been invited. Then, after it was discovered that Elizabeth Costello had a son at Appleton, they were added to the list. He suspects they will be out of place. They will certainly be the most junior, the lowliest. On the other hand, it may be a good thing for him to be present. He may be needed to keep the peace.

With grim interest he looks forward to seeing how the college will cope with the challenge of the menu. If today's distinguished lecturer were an Islamic cleric or a Jewish rabbi, they would presumably not serve pork. So are they, out of deference to vegetarianism, going to serve nut rissoles to everyone? Are her distinguished fellow guests going to have to fret through the evening, dreaming of the pastrami sandwich or the cold drumstick they will gobble down when they get home? Or will the wise minds of the college have recourse to the ambiguous fish, which has a backbone but does not breathe air or suckle its young?

The menu is, fortunately, not his responsibility. What he dreads is that, during a lull in the conversation, someone will come up with what he calls The Question – 'What led you, Mrs Costello, to become a vegetarian?' – and that she will then get on her high horse and produce what he and Norma call the Plutarch Response. After that it will be up to him and him alone to repair the damage.

The response in question comes from Plutarch's moral essays. His mother has it by heart; he can reproduce it only imperfectly. 'You ask me why I refuse to eat flesh. I, for my part, am astonished that you can put in your mouth the corpse of a dead animal, astonished that you do not find it nasty to chew hacked flesh and swallow the juices of death wounds.' Plutarch is a real conversation-stopper: it is the word juices that does it. Producing Plutarch is like throwing down a gauntlet; after that, there is no knowing what will happen.

He wishes his mother had not come. It is nice to see her again; it is nice that she should see her grandchildren; it is nice for her to get recognition; but the price he is paying and the price he stands to pay if the visit goes badly seem to him excessive. Why can she not be an ordinary old woman living an ordinary old woman's life? If she wants to open her heart to animals, why can't she stay home and open it to her cats?

His mother is seated at the middle of the table, opposite President Garrard. He is seated two places away; Norma is at the foot of the table. One place is empty – he wonders whose.

Ruth Orkin, from Psychology, is telling his mother about an experiment with a young chimpanzee reared as human. Asked to sort photographs into piles, the chimpanzee insisted on putting a picture of herself with the pictures of humans rather than with the pictures of other apes. 'One is so tempted to give the story a straightforward reading,' says Orkin – 'namely, that she wanted to be thought of as one of us. Yet as a scientist one has to be cautious.'

'Oh, I agree,' says his mother. 'In her mind the two piles could have a less obvious meaning. Those who are free to come and go versus those who have to stay locked up, for instance. She may have been saying that she preferred to be among the free.'

'Or she may just have wanted to please her keeper,' interjects President Garrard. 'By saying that they looked alike.'

'A bit Machiavellian for an animal, don't you think?' says a large blond man whose name he did not catch.

'Machiavelli the fox, his contemporaries called him,' says his mother.

'But that's a different matter entirely – the fabulous qualities of animals,' objects the large man.

'Yes,' says his mother.

It is all going smoothly enough. They have been served pumpkin soup and no one is complaining. Can he afford to relax?

He was right about the fish. For the entrée the choice is between red snapper with baby potatoes and fettucine with roasted eggplant. Garrard orders the fettucine, as he does; in fact, among the eleven of them there are only three fish orders.

'Interesting how often religious communities choose to define themselves in terms of dietary prohibitions,' observes Garrard.

'Yes,' says his mother.

'I mean, it is interesting that the form of the definition should be, for instance, "We are the people who don't eat snakes" rather than "We are the people who eat lizards". What we don't do rather than what we do do.' Before his move into administration, Garrard was a political scientist.

'It all has to do with cleanness and uncleanness,' says Wunderlich, who despite his name is British. 'Clean and unclean animals, clean and unclean habits. Uncleanness can be a very handy device for deciding who belongs and who doesn't, who is in and who is out.'

'Uncleanness and shame,' he himself interjects. 'Animals have no shame.' He is surprised to hear himself speaking. But why not? – the evening is going well.

'Exactly' says Wunderlich. 'Animals don't hide their excretions, they perform sex in the open. They have no sense of shame, we say: that is what makes them different from us. But the basic idea remains uncleanness. Animals have unclean habits, so they are excluded. Shame makes human beings of us, shame of uncleanness. Adam and Eve: the founding myth. Before that we were all just animals together.'

He has never heard Wunderlich before. He likes him, likes his earnest, stuttering, Oxford manner. A relief from American self-confidence.

'But that can't be how the mechanism works,' objects Olivia Garrard, the president's elegant wife. 'It's too abstract, too much of a bloodless idea. Animals are creatures we don't have sex with – that's how we distinguish them from ourselves. The very thought of sex with them makes us shudder. That is the level at which they are unclean – all of them. We don't mix with them. We keep the clean apart from the unclean.'

'But we eat them.'The voice is Norma's.'We do mix with them. We ingest them. We turn their flesh into ours. So it can't be how the mechanism works. There are specific kinds of animal that we don't eat. Surely those are the unclean ones, not animals in general.'

She is right, of course. But wrong: a mistake to bring the conversation back to the matter on the table before them, the food.

Wunderlich speaks again. 'The Greeks had a feeling there was something wrong in slaughter, but thought they could make up for that by ritualizing it. They made a sacrificial offering, gave a percentage to the gods, hoping thereby to keep the rest. The same notion as the tithe. Ask for the blessing of the gods on the flesh you are about to eat, ask them to declare it clean.'

'Perhaps that is the origin of the gods,' says his mother. A silence falls. 'Perhaps we invented gods so that we could put the blame on them. They gave us permission to eat flesh. They gave us permission to play with unclean things. It's not our fault, it's theirs. We're just their children.'

'Is that what you believe?' asks Mrs Garrard cautiously.

'And God said: Every moving thing that liveth shall be meat for you,' his mother quotes. 'It's convenient. God told us it was OK.'

Silence again. They are waiting for her to go on. She is, after all, the paid entertainer.

'Norma is right,' says his mother. 'The problem is to define our difference from animals in general, not just from so-called unclean animals. The ban on certain animals – pigs and so forth – is quite arbitrary. It is simply a signal that we are in a danger area. A minefield, in fact. The minefield of dietary proscriptions. There is no logic to a taboo, nor is there any logic to a minefield – there is not meant to be. You can never guess what you may eat or where you may step unless you are in possession of a map, a divine map.'

'But that's just anthropology,' objects Norma from the foot of the table. 'It says nothing about our behaviour today. People in the modern world no longer decide their diet on the basis of whether they have divine permission. If we eat pig and don't eat dog, that's just the way we are brought up. Wouldn't you agree, Elizabeth? It's just one of our folkways.'

Elizabeth. She is claiming intimacy. But what game is she playing? Is there a trap she is leading his mother into?

'There is disgust,' says his mother. 'We may have got rid of the gods but we have not got rid of disgust, which is a version of religious horror.'

'Disgust is not universal,' objects Norma. 'The French eat frogs. The Chinese eat anything. There is no disgust in China.'

His mother is silent.

'So perhaps it's just a matter of what you learned at home, of what your mother told you was OK to eat and what was not.'

'What was clean to eat and what was not,' his mother murmurs.

'And maybe' – now Norma is going too far, he thinks, now she is beginning to dominate the conversation to an extent that is totally inappropriate – 'the whole notion of cleanness versus uncleanness has a completely different function, namely, to enable certain groups to self-define themselves, negatively, as elite, as elected. We are the people who abstain from A or B or C, and by that power of abstinence we mark ourselves off as superior: as a superior caste within society, for instance. Like the Brahmins.'

There is a silence.

'The ban on meat that you get in vegetarianism is only an extreme form of dietary ban,' Norma presses on; 'and a dietary ban is a quick, simple way for an elite group to define itself. Other people's table habits are unclean, we can't eat or drink with them.'

Now she is getting really close to the bone. There is a certain amount of shuffling, there is unease in the air. Fortunately, the course is over – the red snapper, the fettucine – and the waitresses are among them removing the plates.

'Have you read Gandhi's autobiography, Norma?' asks his mother.

'No.'

'Gandhi was sent off to England as a young man to study law. England, of course, prided itself as a great meat-eating country. But his mother made him promise not to eat meat. She packed a trunk full of food for him to take along. During the sea voyage he scavenged a little bread from the ship's table and for the rest ate out of his trunk. In London he faced a long search for lodgings and eating houses that served his kind of food. Social relations with the English were difficult because he could not accept or return hospitality. It wasn't until he fell in with certain fringe elements of English society – Fabians, theosophists, and so forth – that he began to feel at home. Until then he was just a lonely little law student.'