Marina Warner is exploring the structure of imagination through a historical eye, focusing on dark monsters as well as jolly fantasy creatures. Harriet Swain reports

In her mind's eye, Marina Warner, writer, art critic and cultural historian, sees the world of the imagination as a world of shadows. Since before the Middle Ages, it has been a place peopled with ghosts, ghouls and images of death and despair. It could be a more cheerful spot, she says. But, on the whole, we have used our imaginations to peer into the dark.

Warner has provided a glimpse through this gloom in her contribution to this year's Darwin lectures, which take as their theme structures of various kinds. Warner has plumped for the structure of the imagination. In her lecture, to be delivered in Cambridge today, she begins by considering different charts, drawn up through history in an attempt to show how the mind's structure was composed. Since Aristotle, no one has been able to decide where to plot the imagination in any map of the mind. This, Warner says, reflects just how hazy our knowledge of it is.

Take the 17th-century writer Robert Fludd, who presented a map of the head showing one of the four characteristics of the mind to be the imaginative soul. He saw this as a projection of the world, but in shadow - a continuation of his Neo-Platonist ideas. "What interested me about this was that it implies that the world seen by the rational soul is real," says Warner, a visiting fellow at Trinity College, Cambridge. "There followed the idea that there is a literal relationship between how we think of imaginary perception and shadow. Fantasies are expressed in terms of shadow."

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She sees this demonstrated through the invention of the camera obscura, a darkened box with a hole for projecting images onto a screen. The camera closely resembled models of the mind conceived at the same time. In his Neo-Platonist study of consciousness and the universe, Utriusque Cosmi, written in 1629, Fludd described the mind's ability to summon images that were distinct from reality. His illustration of this faculty - the oculus imaginationis, or eye of the imagination - showed a single eye in the middle of the head, positioned exactly over the imaginative soul.

The diagram looks like a prophecy of cinema, showing images back-projected from this eye onto a kind of screen at the back of the skull.

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Warner sees a paradox in the rational optical instrument of the camera, devised by scientists, being used to project fantastical images of things that lie beyond sight. The idea of making phantasms project onto a screen led, she believes, to the invention of cinema.

But she has identified influences from other kinds of shadows, too. For her, "cinema is born out of the gothic". The borders defining the imagination have been drawn to focus on the supernatural. "People do not think of the imagination as a faculty of cognition," Warner says. "They do not think of it as something that reflects the world." Instead, they find in it a world of shadows that expresses the Gothic tradition. The literature of fantasy and the ideas behind imaginative works of art tend to focus on the dark side of life, depicting gloom, melancholia, sin and transgression.

At the same time, these imaginings are based on symbolic structures derived from visible phenomena and the meanings invested in them by classical and Christian belief systems. An illustration in the 1671 edition of The Great Art of Light, written some years earlier by the Jesuit Athanasius Kircher, shows the first magic lantern projecting onto the wall the image of a soul burning in the fires of purgatory. Kircher, one of the first to mechanise the camera obscura, drew analogies between it and the workings of a sick mind. His favoured images were those of tormented souls and Death with his scythe.

This morbid theme continued in Phantasmagoria, the first public entertainments to use optical illusion, produced in Paris at the end of the 18th century by the Belgian Gaspard-Etienne Robertson. Robertson took over a convent that had been dispossessed during the French Revolution and draped it with Egyptian hieroglyphics. He specialised in projecting images onto smoke and he built a track that allowed him to move his projector towards and away from the screen. When he pulled the projector back, the picture became bigger. By drawing back rapidly, he could make horrific images of death appear to plunge into the audience, leaving them terrified.

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Phantasmagoria was thought to have influenced Francisco de Goya, who was producing his black paintings in Spain at about the same time. Many of Goya's images were savage, despairing: showing scenes of witchcraft and monsters. Like that of the Romantics, his work seems to show reason and imagination in opposition to each other. The tradition has continued through late 19th-century spirit photography to horror films such as The Thing. "There is a tendency to see the imagination as a faculty that apprehends the dark," says Warner.

But it does not have to be this way. Look at the Teletubbies, she says. They are optical creatures with screens in their tummies and aerials on their heads. But they represent a reversal. Despite being fantasy figures, they are happy, jolly creations, a long way from the gothic monsters of the past. It is the images projected on to their screens, in Teletubby land, that represent the real world and real children.

"The imagination does not necessarily have to be dark," Warner says. "It is not necessarily a tenebrous place. It has that capacity and tendency, but it is not its character." On the other hand, she says, happiness and pleasure are much harder to imagine. "Paintings of hell are full of incredible invention," she says. "Paintings of heaven tend to be a bit similar to each other."

Marina Warner's lecture, "The structure of the imagination", Darwin College, Cambridge, 6pm.

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