“I shall define phobia to be a fear of an imaginary evil,” he wrote, “or an undue fear of a real one.” He listed 18 phobias, among them terrors of dirt, ghosts, doctors and rats.

They came to see phobias as lurid traces of our evolutionary and personal histories, manifestations both of animal instincts and of desires that we had repressed.

They identified dozens of irrational fears, among them fears of public spaces, small spaces, blushing and being buried alive (agoraphobia, claustrophobia, erythrophobia, taphephobia).

To be diagnosed as a specific phobia, a fear must be excessive, unreasonable, and have lasted for six months or more; and it must interfere with normal life.

This makes it difficult to measure their prevalence, but recent studies suggest that one woman in 10 experiences a specific phobia, and one man in 20.

Freud proposed that a phobia was a suppressed dread or desire displaced on to an external object.

“Phobia particularises anxiety,” observes the literary scholar David Trotter, “to the point at which it can be felt and known in its particularity, and thus counteracted or got around.” Evolutionary psychologists argue that many phobias are adaptive: our fears of heights and snakes are hardwired in our brains to prevent us from falling from heights or being bitten by snakes; our disgust at rats protects us from disease.

But phobias may also seem more common in women because the social environment is more hostile to them, or because their fears are more often dismissed as irrational.

All phobias are cultural creations: the moment at which each one was identified – or invented – marked a change in how we thought about ourselves.

A fear of clowns, known as coulrophobia, became prevalent in the US in the 1980s, after newspapers published pictures of the serial killer John Wayne Gacy dressed in a clown suit.

For them, the trouble expressed by a phobia was not personal but communal.

Tetraphobia, or a fear of the number four (tessares in Ancient Greek), is common in East Asian countries, because in several languages (among them Mandarin, Korean and Japanese) the sound of the word “four” is very similar to the sound of the word “death”.

An aversion to clusters of holes or bumps was identified as a phobia in 2003, when an image of a seemingly maggot-infested female breast was circulated on the internet.

The word was a translation of the German eisenbahnangst, or “iron-road-angst”, into the Greek sideros (iron) and dromos (track) and phobia (fear).

Freud thought his railway phobia began on an overnight train trip from Leipzig to Vienna when he was two.

He speculated that he had seen his mother naked on this journey, and had developed the phobia by displacing on to the train both his excitement – “my libido was stirred up towards matrem” – and his fear that his father would punish him for his desire.

Hypnophobia – from the Greek hypnos, sleep – is a morbid fear of sleep, usually caused by a terror of dreams or nightmares.