Among the numerous gems of eloquence with which Barack Obama strewed his path to the White House, one that has already gained a footing in collections of quotations is this: "We have been warned against offering the people of this nation false hope. But in the unlikely story that is America, there has never been anything false about hope."

Because the resonant ideas of hope and its necessary correlative, change, inspired so many Americans to vote who had never voted before, especially among the young and hitherto disaffected, an immediate reaction among observers is to ask: given the inevitability that not all, or perhaps even many, of the hopes that Obama has raised can be realised, surely he has taken a great risk in loading his presidency with such expectation? Why is it that people fail to realise how unlikely it is that all the hopes generated by his campaign will come to fruition?

These are good questions because they are prompted by one of history's clearest lessons: that almost all political endeavours end in failure and disappointment, whether or not they start in a joyous riot of welcome and relief. A minor forerunner of Obama's capture of the White House is Tony Blair's election victory in 1997, which reminds us of a painful truth: the higher the starting point, the steeper the eventual fall.

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Obama inherits a disastrous legacy of wars, financial crisis and poisoned alliances, an America still divided despite the signal achievement of his victory as a black man, and surrounded by an unstable and uncertain world. Failure in some or most of the tasks that face him is a near-certainty. Yet he is launched on those tasks on a tsunami of expectation. What place can the euphoria of hope have in this, other than to blight him eventually?

The answer lies not in whether the hopes will materialise in future, but in the catharsis of having hope in the present. Along with something to do and someone to love, having hope is one of life's essentials. It is an obvious but important fact that genuine hopelessness is a destructive, suicidal, terminal state of mind for an individual, and equally corrosive for a society. Even unrealistic hope is enough to get people out of bed in the morning, and to try again after failure or rejection.

Hope is a psychological necessity, and perhaps it is a distinctive mark of humanity too; for alone among animals, human beings have foresight which, informed by memory, would seem to give a truly rational individual good grounds for steeling himself against hope as much as possible – and yet most of us do exactly the opposite most of the time.

In describing a second marriage as the "triumph of hope over experience" the inimitable Dr Johnson exploited the central fact about hope: that it is an emotion, and moreover one of those emotions, such as fear, greed, desire and love, that trump rationality every time. Dr Johnson's contemporary, the philosopher David Hume, argued that reason cannot move people to act, only emotion can; unless we felt strongly enough to exert ourselves in one direction rather than another, we would do nothing, but would suffer the fate of Buridan's Ass, which starved to death between two bales of hay because, having no reason to eat from one bale rather than the other, it could start on neither.

Hope, however, is one of the strongest of motivating emotions. As a non-rational sentiment it has no interest in weighing the likelihood of success; it just adds more hope, that things will work out as hoped; that "this time things will be different"; that the very strength of hope will itself surmount obstacles and alter realities. In the not infrequently self-fulfilling way of emotions, hope is thus its own best hope, because in sufficient quantity it can indeed be powerful. If proof were required, one has only to note that the hopes generated by Barack Obama's candidacy carried it to success.

The hard realities of office over Obama's coming four years are of course a different matter. Even if hope continues to run at fever pitch, which it will not, it cannot by itself be enough. Others have hopes too: Obama's opponents, among them those who positively hate him and hate the fact that he has been elected, hope that he will fail, and some doubtless even hope that he will be assassinated. Like the intoxicating hopes raised by a liberal, eloquent, new force in American life, these perverse hopes also help some to get out of bed in the morning. Hope is no respecter of principle or morality.

The main point, though, is that although hope by its nature is a forward-looking emotion, its real effect lies in the present: it is what motivates and encourages now, it is what makes a difference to how the world seems to us even in the middle of difficulties. Those cheering multitudes in Chicago's Grant Park and the streets of other American cities, who had both hope and a vote, to say nothing of delighted people around the world who had no vote but only hope, were reaping hope's benefits in the moment.

The future was then only relevant as the site of dreams. One hopes that the fact that they hoped will sustain them as euphoria fades.

A C Grayling is Reader in Philosophy at Birkbeck College, London