But were its original drafters in 1776 correct in using the word 'happiness'? It is a brilliantly coloured, vivacious word; an ambitious, even garish, and, as it has turned out, doom-laden word. They might have done better to opt for 'contentment', even just 'equilibrium'.
Few of those in pursuit of happiness stop to define it. They know only that they are entitled to it; their constitution says as much. This belief is not confined to Americans: we all share it. The 20th century prizes happiness as no other century has done. It is what we think we are looking for as we race after a better job, more money, husband, a better husband, a flat, an upgraded home, a second home, country house, a car, a second car . . . the rewards become greater, the pursuit more relentless, the frenzy madder. In the course of it, happiness gets lost. Are the rich happier than other people? Certainly not - although they are compensated, no doubt, by being unhappy in comfort.
The revolutionary idea that happiness is everyone's right was implanted by the United States and amplified by the great social reformers of the 19th century. Until then, most people would not have believed for a moment that life was anything other than nasty, brutish and short. Comfort was the province of the rich; for everyone else, happiness could be only a brief ray of light that accompanied, with luck, marriage and the birth of a first child. The most haunting words Thoreau ever wrote were: 'The mass of men lead lives of quiet desperation.' The most haunting thought is that most people still do.
Happiness has been confused with wealth, and the pursuit of happiness has become materialism: the amassing of money at any cost. But if money can't buy happiness, does one pursue it? Not, I think, headlong but obliquely, and not in the expectation of finding it.
It is courting disaster to try to define happiness, but here goes . . . Happiness is any situation in which the basic human requirements - for shelter, warmth, food, privacy, companionship, work and relaxation - are met. This can be combined with living at a humble and frugal level, since the addition of more choice or greater luxury involves extra effort, more complications, and greater risk of disturbance. Thoreau again: 'Our life is frittered away by detail . . . Simplify, simplify.'
I have been giving this matter of happiness some thought because, for the moment, I think I have it. For the past three weeks I have been living in a house half - no, a third - the size of my London flat. This house contains not a single machine: except, I admit, a word processor and a fax machine, for I can no longer write without them. I do everything by hand. (And already my hands look different as a result: no longer rich, idle hands but rough working ones.)
I spend a fraction as much money as usual on food and drink, since we live mainly on good nourishing soup. My choice of what to wear is dictated solely by how cold it is. We see almost no one: the arrival of the postman is an event. I get up and go to bed early, and sleep soundly. The reason for this retirement and frugality is that I am finishing a book.
It helps, of course, that I am in the depths of the countryside, that my front door opens on to a view of startling beauty, and that the weather has been mild and dry. But those are additions to my happiness; I believe I would be perfectly content without them.
You may say, reading this, that I am like a child playing house; leave me here for ever and I would soon tire of the game. I do not think so, because the satisfaction is so great. I have lost urgency, anxiety, crowds and too many choices; I have gained concentration, clarity and calm. I live in equilibrium.
If the Declaration of Independence had asserted the right to equilibrium, the US might be a different, and happier, place.