This is the last in a series of eight articles about love. Last week I considered what it means to be 'in love' and asked whether it is possible for a long-standing couple to stay in love.

I agreed with the philosopher Andre Comte-Sponville that the essence of romantic love (eros) is want. In other words, to be 'in love' is to be consumed by a passionate longing for someone.

Hence it is impossible for two people to stay 'in love' when they have lived together for a long time. Why? Because when want is satisfied, it ceases to be want. We cannot continue to crave that which we already possess.

Love and liberty

The Scottish philosopher David Hume (1711-1776) once wrote an essay about marriage, polygamy and divorce. In it, he remarked that romantic love requires freedom in order to survive.

'Love is a restless and impatient passion, full of caprices and variations: arising in a moment... and suddenly extinguishing after the same manner. Such a love requires liberty above all things.' (From Hume's essay, Of Polygamy And Divorces)

Typically, when two people fall in love, they are free and unattached. They live in different houses, lead separate lives and are free to give their hearts to whoever they choose.

Romantic love flourishes under these circumstances. The time lovers spend apart makes the time they spend together all the more precious; and love is all the sweeter when it is given freely and spontaneously.

But once two people commit to being 'a couple' (typically by marrying), they are no longer free to live and love as they please.

Their actions and affections become bound by constraints - constraints that are entered into voluntarily, but constraints nonetheless.

Eros cannot flourish under these circumstances. Duty, responsibility and routine are inimical to passion. In time, the heart palpitations and giddy intoxication of new love must give way to something more serene.

Marital friendship

But although eros cannot survive without liberty, friendship can.

David Hume writes: 'But friendship is a calm and sedate affection, conducted by reason and cemented by habit; springing from long acquaintance and mutual obligations; without jealousies or fears, and without those feverish fits of heat and cold, which cause such an agreeable torment in the amorous passion.'

Unlike eros, friendship thrives under constraint. The very stuff that chokes the life out of passion (duty, responsibility and routine) causes friendship to flourish and bloom.

'Friendship... never rises to such a height as when any strong interest or necessity binds two persons together, and gives them some common object of pursuit.'

This is why successful marriages are not characterised by swoons, sighs and palpitations of the heart - though they will have had their share of these things at the beginning. They are characterised by care, consideration and tenderness.

The happy couple, says Comte-Sponville, 'have managed to transform the passion and ardour they had in the beginning into joy, gentleness, gratitude and trust, into happiness in being together.'

In other words, into friendship.

Friends and lovers

Does this mean that desire has no place in a long-standing relationship?

Not at all. It merely changes.

The essence of eros is want. When we are 'in love', the desire we feel is urgent and hungry - even selfish.

"At first we love only ourselves,' says Comte-Sponville, 'the lover throws himself on the loved one like the newborn upon his mother's breast - or the wolf upon the lamb.'

But as eros subsides and friendship increases, a couple learn to love less selfishly. Love-making becomes less urgent, but more generous; and although passion subsides, pleasure remains.

Successful couples manage to unite friendship with desire.

In fact, the theologian and philosopher St Thomas Aquinas believed that the friendship between husband and wife is the greatest friendship there can be: 'For they are united not only in the act of fleshly union... but also in the whole range of domestic activity.'

The final word goes not to Comte-Sponville, David Hume or St Thomas Aquinas, but to leather-wearing 1970s rocker Suzi Quatro.

From her song You Are My Lover:

'You are my lover and my best friend.

I think I've found the perfect blend.'