Depression is said to cost the economy I don't know how many £million a year because of demands on the National Health Service and lost working days - to say nothing of personal suffering and the strain on victims' families. The problem doesn't seem to go away and may even be growing. But what exactly is depression?

Contrary to what many people believe, depression is not an illness. It is simply a mood that stems from an unfulfilled longing for something we cannot have or do. It is a natural and healthy response to an inability to cope with circumstances is, although we usually have difficulty seeing it that way at the time, particularly if we are unaware of the underlying causes.

So long as the real causes remain concealed, the longing persists and the mood is likely to intensify and promise nothing other than endless despair. At that point it is likely to be described as 'depressive illness' or, in medical terminology, 'clinical depression'. This involves specific disorders, such as frequent, irritable behaviour; loss of appetite; significant changes in body weight; disturbed sleep patterns; trembling; loss of physical and/or mental energy; a diminished ability to think or concentrate; irrational and debilitating attacks of panic; constant anxiety; a sense of isolation verging on paranoia; unjustified feelings of worthlessness — perhaps culminating in thoughts of suicide. The actual extent to which these disturbances warrant the term illness is left to the personal judgement of the doctor.

There is then a risk that the primary cause will be attributed to a somatic change of some kind, despite there being no biological evidence for such a hypothesis. Hence, attempts to 'cure' it by methods unrelated to the real causes provide little more than short-term relief.

So, when we are depressed, can we avoid becoming clinically depressed? Many people try to avoid it by taking 'positive' action, such as work, hobbies, sport, social activities; or perhaps shopping, travel, food and mood-altering drugs, legal, proscribed, or perhaps prescribed. These undoubtedly 'take one's mind off' the immediate problem and, if the mood is mild enough, it will probably go away of its own accord. If it does not, such activities are likely, in the same way as medical treatments, to aggravate the mood by helping to conceal the real causes.

Having frequently been depressed myself, sometimes to the point of feeling I am trapped in a total void and unable to do anything about it, I have concluded that efforts to lift the mood are in fact counter-productive. Instead. I try to stay as calm as possible and let the mood run its course. Like all animals, we have an in-built tendency to revert to health and emotional equilibrium whenever the circumstances are favourable. In relation to physical health, we call this homoeostasis or the vis medicatrix naturae. Thus, the more we panic and try to rid ourselves of what is undeniably a very distressing state, the more we exhaust the store of emotional energy that could otherwise help provide relief. Like any mood, depression can neither be induced nor dispersed at will. But if it is properly managed, we need do little more than wait for it to pass — as moods do sooner or later. The 'time out' that depression encourages is an opportunity for us to take stock of our situation.

Meanwhile, we can remind ourselves that a half-empty glass is also a half-full one, and that the mood is likely to pass, because it always has before. Although the argument is irrational, and hardly convincing when there is not even a glimmer of light at the end of the tunnel, it is no less rational than the belief that the mood will never pass.

Although the true causes of depressed moods are seldom immediately obvious, I guess that they occur mostly when we have dangerously overloaded our capacity to cope; and that they stem from an undetected need to slow down and do just enough to ensure we remain physically healthy, so that we can at least stop the overload increasing. Perhaps the mood is an unconscious attempt to tell ourselves something we do not want to hear, providing an opportunity to learn more about ourselves, and alerting us to the risk of clinical depression.

Simply by 'putting one foot in front of the other' and concentrating our entire attention on the immediate task, sufficient energy eventually comes, apparently from nowhere, unless there is something that prevents it. Thus, the downward spiral is arrested, and nature gets a chance to effect its cure.

Urged on by social pressures. such as the need to earn a living or see to the needs of one's family, and perhaps the risk of social opprobrium, can we always afford to slow down? The question can best be answered by asking another: what is likely to happen if we do not? Altering one's current way of living is not the same as doing nothing at all, because a complete absence of physical activity only encourages the mind to go round in ever-tightening circles.

There will be time enough to ponder the possible causes and remedies when the mood lifts. Unfortunately, that is also the time when it no longer seems to matter. Consequently, we tend not to bother!

I cannot say, of course, whether my strategy will work for everyone. No two people and their circumstances are precisely the same. The true causes of depressed moods, whether 'clinical' or not, can only be discovered case by case; and I find the notion of an unwanted mood as an illness whimsical.

Discovering the causes is likely to take time, patience, personal courage and perhaps skilled help. But it can be done. How else might the epidemic be halted?