A good idea from ... Stendhal

UNHAPPY love affairs force even the most unreflective people to start thinking. It seems impossible not to write at least a few lines of poetry or a diary after one's heart has been broken. The French novelist Stendhal (1783-1842) observed that he would never have taken up his pen if, at 18, he had known happy love. Fortunately for us, Stendhal was unhappy in love throughout his life, and his most painful passion - for a beautiful, enigmatic Italian woman, Metilde Dembowski - drove him to produce his greatest book, De l'Amour, a mixture of anecdote, analysis and aphorisms, published in 1822.

De l'Amour sets out to understand, with quasi-philosophical detachment, every stage of romantic love, but the author's emotions keep breaking through the rational form of the book. Stendhal said he had to stop writing every few lines in order to weep. (According to Lytton Strachey, Stendhal combined "the emotionalism of a schoolgirl with the cold penetration of a judge on the bench".)

In the most famous passage of De L'Amour, Stendhal puts forward his idea of "crystallisation": the stage, in every great love affair, between identifying someone desirable, and possessing them physically, marked by a wild mixture of hope and despair, during which one's emotions pass from mere liking to love.

"Leave a lover with his thoughts for 24 hours, and this is what will happen. At the salt-mines of Salzburg, they throw a leafless wintry bough into one of the abandoned workings. Two or three months later they haul it out covered with a shining deposit of crystals. The smallest twig, no bigger than a tom-tit's claw, is studded with a galaxy of scintillating diamonds. The original branch is no longer recognisable. What I call crystallisation is a mental process which draws from everything that happens new proofs of the perfection of the loved one."

Stendhal stated, as though it was an inviolable psychological law, that one always had to leave a person in doubt for them to fall in love. Without doubt there could be no crystallisation. Doubt made the lover lose perspective, every thought and conversation would bring him back to his beloved.

"You hear a traveller speaking of the cool orange groves beside the sea at Genoa in the summer heat: Oh, if you could only share that coolness with her! One of your friends goes hunting, and breaks his arm: wouldn't it be wonderful to be looked after by her ... "

Stendhal knew how silly this sounded, but his theory captured the insane discrepancy between fantasy and reality; in the salt-mine, the smallest twig could end up being covered in what looked like diamonds.

Then again, Stendhal was also sympathetic to the humiliating madness of love. "If you have never experienced an unhappiness beyond worry over a lawsuit, or not being witty enough when you were taking the waters at Aix last season," he explained in his introduction, then his book would not be for you.

In his memoirs he summed up: "My normal state has been that of an unhappy lover, who loves music and painting deeply. Daydreaming is what I like to do best of all." Literature is the richer for his many griefs.