Many people dream of a partner for life but bolt when the intoxication of a new affair wanes. As Valentine's Day approaches, Julian Baggini ponders the nature of romantic love

Iwas at a wedding recently, listening to a recitation of Shakespeare's Sonnet 116, when I found myself wondering: does love really not alter when it alteration finds? Not the most apposite thought for such a romantic occasion, perhaps, but only to be expected from someone who's spent too much time reading philosophy.

Of course, love frequently does alter if it finds that its object is not a loyal, devoted partner but a cheating, lying cad. However, that was not the kind of transformation I was thinking of. My question was whether the very nature of love has changed over the generations.

Many people seem to think that love isn't what it used to be. Divorce has become normalised, with nearly half of all marriages ending in separation, on average after less than 12 years. The idea of finding one partner for life can now seem quaintly old-fashioned. Indeed, people who marry childhood sweethearts are often not envied but pitied for the narrowness of their experience. Despite the wedding vows that state the contrary, it seems that love is becoming less a lifelong bond than a temporary state to be discarded when it has served its purpose or ceased to function.

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In choosing mates, we have become more like canny consumers than star-crossed lovers. Singletons satiate their desires with snazzy disposables rather than go without or pay too high a price for something longer lasting but less attractive. We all now believe in try before you buy, and prenuptial agreements, formal or informal, are Cupid's own money-back guarantees.

And yet there is something odd about the claim that love is changing. After all, when we look for the words to describe how love makes us feel, we turn to centuries-old poets. This should not surprise us: the fundamentals of human nature do not change wildly from generation to generation. Love is a primal emotion, and although we may find it hard to imagine what it was like to live the life of an Elizabethan playwright, we all know what he meant when he wrote "Love is the most beautiful of dreams and the worst of nightmares". So, although something is clearly changing, a great deal has remained the same. To put it crudely, falling in love is much as it ever was; it's staying in love that has changed.

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Most of the great love literature of the world is about wooing, seduction or overcoming the obstacles that stand between desire and its consummation. Much of the rest is about the heartache of separation. The settled love of long-time partners and spouses rarely plays the muse. Andrew Marvell wrote to his coy mistress, not his loyal wife.

The trouble is that the intoxication of falling in love rarely lasts. A team of Italian psychiatrists at the University of Pavia have even been so precise as to state that such romantic love usually lasts no more than a year. Long-term partnerships are sustained by something else, which varies according to time and place.

But there are no historical or geographical constants in the relationship between falling in love and forming a pair-bond. For instance, although we tend to assume that the former precedes the latter, in many cultures where marriages are arranged falling in love is something you hope to do only after you have wed. In cultures where having a mistress is the norm, falling in love is something that you expect to do with many people other than your spouse, whose main role may be to provide neither love nor companionship but status, security or children.

Historically, marriage has always been an economic, practical or even political arrangement as much as, or even more than, a romantic one. In that respect, modern online and speed daters are, if anything, less cold and calculating than their forebears who haggled over dowries and were rarely irrational enough to marry without thought for practicalities. Like Tevye and Golda in Fiddler on the Roof , asking whether they loved each other long into their marriage would seem bizarre, not a prelude to a marital crisis. The sort of quiet love they "suppose" they must have for each other is something many would now dread: tell your partner that you see them like an old favourite pair of slippers at your peril. Yet for many today the problem of what comes after that initial, burning love has not been solved. Good sex is considered a vital part of the right chemistry with a partner and is widely seen as essential to a solid, lasting relationship. But time and familiarity is more often its enemy than its friend. Evidence is difficult to come across, but stories of couples, even young ones, whose sex life becomes sporadic or non-existent after a few years together are very common - and for many that spells the beginning of the end of the relationship.

There are those who shrug their shoulders at this and say simply that we were not designed to stay with the same person for about 50 years or more, which is what would now be required of a typical couple who wed in their twenties and never divorced. For most of human history human lives have been much shorter, and there would be a very high chance of one or other spouse dying prematurely. Second marriages were common long before divorce became the usual cause.

"Free love" has been tried, and for most it fails. So serial monogamy is now touted as the natural response to a combination of increased life expectancy and the weakening of economic and social imperatives for unhappy couples to stay together. There is surely something to this received wisdom, but it fails to explain two facts about modern love. If serial monogamy is now the natural choice, why do most see it as second best? Finding a partner for life remains the most popular goal, which is why at weddings people do not promise to have and to hold until ennui doth them part. That breaking-up is as much an entrance as an exit is something we try to believe to get over its wretchedness, not a truth we instinctively recognise.

Philosophy is not an obvious place to look for solutions to these dilemmas. There is very little that is rational about falling in love, which is perhaps why the great philosophers of the Western tradition have had so little of interest to say about it. Spinoza argued that "love is nothing but a pleasurable state, joy, accompanied by the idea of an external cause". No wonder you don't hear him quoted at weddings and on Valentine's cards. Bertrand Russell acted as though passion and reason were entirely different and that if he woke up one morning in love with someone different from the day before there was nothing more to be said or done about it.

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Love eludes philosophical elucidation because, in the dichotomy so brilliantly set up by Tom Stoppard in his classic Arcadia, it belongs to the romantic, not the classical, imagination. Yet, as that play showed, the two realms cannot be set entirely apart, and I would suggest that analytical thought can contribute something to the possibility of romantic fulfilment. To bastardise Kant, reason plays a part not because it justifies love, but because it makes a proper place for it.

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The romantic appeals not to how things are as a matter of objective fact but how they feel to us and how we wish them to be. It is therefore no surprise that reason finds it uncongenial. But without any trace of the romantic, human life becomes barren. The trick of reconciling romanticism and reason is to use the latter to keep the former within the realms of the possible, not confine it to the actual or the merely probable.

In the sphere of love, the problem is not that we are hopelessly romantic but that we are romantic about the wrong things. The failed romanticism of the second half of the past century is the one that thought love was all about authenticity, spontaneity and freedom. The idea is that once we threw off the conventions that kept us all sexually oppressed, love would be pure and free. This is a romantic vision in the pejorative sense because it is an overly optimistic vision of how we want things to be. If only it were the case that all you had to do was be true to your feelings and you'd have hang-up free sex in equal and open relationships.

The reality is that if sex is good only when spontaneous there is nothing to be done for couples for whom the effortless spark of lust has become a wet box of matches. For most people to enjoy a lifelong sex life requires periodic renewal, which is not a matter of revisiting first infatuation but moving the relationship on to another level. But this is hard, and sex was not meant to be difficult any more.

The idea that if we simply get in touch with our natural urges we will be free from any "issues" with sex is romantic twaddle of the worst kind. On the contrary, I would expect sex to be a frequent source of unease and internal conflict. The sexual urge is a primitive call to copulation that has little regard for the priorities of the conscious mind, such as for trust, faithfulness, sensible family planning and so on.

Yet romanticism can be rescued. There are two ways of retaining a romantic vision while keeping our feet on firm ground. The first is to embrace serial monogamy, heartache and all, and to take the pain with the passion. It is to believe in the ability to love again and again even after love has ended and perhaps left us in tatters.

The second is to hold on to the ambition for lifelong love but to accept that it is not about effortless, lasting happiness. It is to believe in the possibility of love enduring despite unreliable libidos, hard times and perhaps finding that your partner is not always there for you as he or she should be.

Both these alternatives have in common a realistic acceptance of love's pain and imperfection. Yet they are both properly called romantic visions because they believe in the power of love to overcome obstacles, and they assert its value despite the fact that it is not remotely rational.

The desires for sex, love and companionship have not changed, but the circumstances in which those desires can be expressed and fulfilled have. If the practice of love refuses to alter when it such alterations finds, then it is even more of a fool than it necessarily is already.

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Julian Baggini is editor of The Philosophers' Magazine . His book, Welcome to Grey Town: A Journey in the English Mind , is published by Granta in March.