One view of happiness, often called hedonism, is that happiness is simply a matter of subjective feeling. A happy life is one that maximises feelings of pleasure and minimises those of pain. But it can't be as straightforward as that.

Professor Martin Seligman, author of Authentic Happiness and pioneer of positive psychology, offers the following insight.

In happiness studies, researchers contact people at random during the day and ask how much pleasure or pain they are experiencing at that very moment. The researchers then extrapolate their data and calculate an approximate total for the amount of happiness experienced by that person over the week.

Afterwards, they ask the same people 'how happy was your week?' Time and again, people's retrospective view of their happiness differs greatly from the extrapolated total of experienced happiness. How was your holiday? 'It was great,' you reply, honestly. Yet if the researchers had contacted you at various times on your vacation, you would have reported all sorts of miseries - the sunburn, the squabbling kids, the overpriced drinks, and so on. Which is the more meaningful measure of happiness - what you feel at the time or the retrospective view?

As Seligman points out, when we wish someone a happy life (or a happy childhood, or even a happy week), we are not merely wishing that they accumulate a pile of pleasurable moments, irrespective of how they are distributed across one's life-span. We can imagine two lives that contain the exact same amount of momentary happiness. One life, however, is a story of gradual decline from blissful childhood to miserable old age. The other is the reverse - a tale of gradual improvement. The same amount of happiness but vastly different lives. The difference between the two lives can only be discerned by a retrospective examination of the the life pattern as a whole, not simply by the total in the happiness ledger.

The philosopher Ludwig Wittgenstein's was by all accounts a miserable man, whose life was full of negative emotion, yet his last words were: 'Tell them it was wonderful!' Whose life is it anyway?