A new butcher's opened in Primrose Hill earlier this autumn, and because the shop sells foie gras it has been besieged by animal rights protesters, if only on Saturday afternoons. 'You have blood on your hands,' was one of the taunts aimed at the butchers the other day.

The livers of wild geese and ducks typically double in size as they prepare for migration or the winter ahead. If domesticated and force-fed, their livers can expand six times or more. The fattening of all animals is ancient and persistent, and the making of foie gras is as old as Greece. Ditto, the sacrificial and spiritual significance of the livers of goats, sheep and cattle; the complexion of a liver determined whether the feasting element of a sacrifice would go ahead. If the liver looked unusual then the animal was dispensed with. The cultivation of edible livers has been so systematic and accompanied with such veneration and symbolic force that to call the practice 'inhumane' is, historically speaking, to misrepresent it (which isn't to say the animals don't suffer).

There’s the human dimension, too. Plato thought the liver the home of the lower soul; Galen thought it comforted and warmed the stomach; Zeus punished Prometheus for giving men fire by chaining him to a rock where his liver was pecked at by an eagle, only for it to regenerate over night and be fed on again the next day. Savoured in the past less for its taste than its symbolic value, the liver is the only human organ capable of regenerating itself.

In Memories of Gascony, the chef Pierre Koffmann describes the way his grandmother, Camille Cadeillan, went about preparing the livers of her poultry. She kept one hundred ducks but only a few geese, as they required too much feed.

Only mature ducks were force-fed, and all of Camille's ducks were at different stages of development. Nevertheless she knew her flock like the back of her hand, and remembered at a glance the precise age of each duck. She organised the fattening and slaughtering of the batches. The first batch, which consisted of about fifty birds, had to be ready for selling in Fleurance market on the last Tuesday before Christmas. The process of fattening took precisely three weeks, so she was able to calculate the exact day on which it should begin.

The practice Koffmann describes is near-identical to the one outlined by Charles Estienne 500 years earlier in Maison rustique, a popular book in 16th and 17th-century France, translated into English in 1600. ‘They must given them thrice a day barley and wheat meal tempered with water and honey,’ Estienne wrote, ‘for the barley make the flesh white and the wheat maketh them fat and maketh a great liver.’ Rearing geese and ducks, he observed, also supplied the farm with its quills.

At the Cadeillans farm, the ducks selected for fattening were put in a dark room and fed twice a day. If it wasn’t done correctly, Koffmann says, the duck might suffocate:

All through the feeding time, Camille's hand would run up and down the duck's throat, softly and judiciously feeling it and pressing to see how the the bird was taking the maize, whether the funnel was in the right place and whether the crop was full and the duck could take no more.

Twenty-one days later the ducks were ready to be slaughtered. Koffmann's grandmother and one of her friends – it was a two-person job — slit throats and collected blood. Next there was the time-consuming but important process of plucking the down for pillows, cushions and eiderdowns:

In the evening sitting round the fire, they would heat little flat irons near the embers and pass them carefully over a damp cloth laid across the white down of the ducks bodies. After a gentle warming, the down came away quite easily... The sight of four women sitting round a fire and ironing some thirty to forty ducks as though they were so many handkerchiefs was certainly an unusual one.

The ducks were sold at the market, or to villagers who had pre-ordered. A doctor in the nearby village typically wanted ten, the lawyer half a dozen, the postman four. The birds the family kept for themselves were stored in the well until they were cooked. 'The foie gras,’ Koffmann says, ‘was of such good quality that hardly any fat came out of it.’

The process didn’t stop with feeding, killing, plucking and cooking:

Each duck killing was followed by days when my grandmother prepared the bottles of rendered duck and goose fat which were kept in the dining room cupboard and would play such an important part of her cooking during the coming year. She made large jars of confits de canard, or preserved duck, which was equally important for her cooking.

Rearing geese and ducks for foie gras is illegal in Britain, though as the protesters on Primrose Hill know, it’s fine to sell livers from elsewhere. If there are laws on what you can and can’t do when fattening geese and ducks, why has it proved so hard to impose restrictions on industrially manufactured sugars and fats that make millions of people obese? What is it about the goose?

Charles Estienne, printer to the king of France in the mid-16th century and a renowned anatomist, ended his chapter on the importance of geese with another of the bird's less obvious virtues: 'The dung of geese dried, powdered, and taken in the morning with one dram of white wine, doth throughly cure jaundice, if it be continually used for the space of nine days.’