One can be greedy about food, money, attention, possessions, even love. That it is hard to stop being greedy shows that it is an involuntary thing. Nobody chooses to be greedy; nobody works out how to do it and then practises. The reasons for it lie outside conscious control, yet they exert powerful control.

There are three main reasons. One is that greed is a response to earlier lack. A poor childhood can make some people frugal about everything all their lives, even if they become rich. But it can have the rebound effect of making people incontinent in eating or buying once the chance arises. They are compensating for what they did not have when young; but because they can never fill the belly of the hungry child they once were, they cannot stop themselves in the present.

The second cause of greed is the opposite: this time it is a spoilt childhood, in which most desires are readily gratified. The inability to defer fulfilment is an undermining weakness. The habit of filling one's mouth with food, one's room with toys, one's life with amusements, is a recipe for lack of restraint. But whereas the person made greedy by early poverty is usually energetic in gathering what he wants, the person made greedy by early excess tends to lack that energy, instead passively waiting for someone else to stuff the cream cake into his mouth.

The third source of greed is that other sin, envy, or its cousin, covetousness: wanting what the neighbour has. This greed is inflamed and kept at a chronic level by advertising or by the goods attractively arrayed in shop windows — all inducement to possess and consume.

But to be greedy is to be eternally poor, because a greedy person can never have enough; but to be satisfied with enough is always to be rich, and content. And, usually, slimmer.

A. C. Grayling is Professor of Philosophy at Birkbeck, University of London

Times recommends

• The grim reality of life as an air stewardess
• 50 of the world's best design blogs
• Video: Georgia Jagger on fashion and her mum's clothes