Modern Times began in the 18th century, the so-called "Age of Reason", when deep, immemorial shadows of ignorance and fear started to be driven back into the odd corners where some of them linger to this day. Perversely, however, deprived of the consolations of superstition, people soon discovered a need to torment themselves afresh with imaginary terrors, and the Gothic - dark side of the moon to the Enlightenment sun - was called into being.
Naturally, compared to the real threats that had lurked in the ancestral forests, there was something inescapably daft and self-conscious about these new anxieties. A vein of unintended hilarity - satirised by Jane Austen in Northanger Abbey - runs through Gothic like the facetious legend in a stick of seaside rock. Naturally, too, we moderns are well aware that everything comes down to sex in the end (the Marquis de Sade is as central to the Enlightenment as David Hume) and, sure enough, weird sex is as character- istic a Gothic trope as the weird sisters. Today, this combination of scary, sexy and silly is as irresistible as it ever was - think of Buffy the Vampire Slayer. A new exhibition at Tate Britain, "Gothic Nightmares: Fuseli, Blake and the Romantic imagination", looks certain to be one of the crowd-pleasers of 2006.
Just because it was daft didn't mean it hadn't a serious side, though. Gothic satisfied a real need in Georgian society, providing a sort of virtual environment in which the unthinkable could be contemplated in relative safety. Henry Fuseli (1741-1825) is the central figure of the exhibition, but "Gothic Nightmares" is not a one-man show. In the popular new fashion of "Turner Whistler Monet" and "Degas, Sickert and Toulouse-Lautrec", the curators have juxtaposed Fuseli's work with that of some of his great contemporaries; it appears here alongside the visionary, apocalyptic paintings of William Blake (1757-1827) and the bit-ter, scabrous political cartoons of James Gillray (1757-1815).
One exciting feature of the show is a reconstruction of a "phantasmagoria". This was a sort of wild ancestor of the cinema - a souped-up slide show with sounds, smells and shadow play that was a Europe-wide sensation around 1800. Some of Fuseli's pictures borrow classical and Shakespearean motifs, but many do not, and even if they now all look like costume dramas, we should remember that when they first appeared, many showed people in contemporary dress enacting urgently contemporary themes. In a good phrase, Marina Warner identifies Fuseli's portrayal of women as a "decadent concoction of vice, cosmetics, revolution and misogyny".
In its day, Gothic played on contrary feelings of fascina- tion and repulsion, and offered guilt-fuelled outpourings and Oedipal dramas a hundred years before Freud. Dreams were at the heart of it, and Fuseli - whose Nightmare (1781) is an important Gothic image and the star turn of the exhibition - was popularly supposed to eat raw pork at bedtime to induce them. In truth, however, neither he nor the Gothic needed artificial stimulants to bring the black stuff bubbling to the surface.
The Nightmare shows a girl in a diaphanous nightgown sprawled on a mattress. Tossing and turning, she has thrown off her covers and is hanging half off the bed. Her face, trapped in sleep, registers horror. A livid, eyeless horse's head - the "night-mare" of the title - thrusts through her bed-curtains and an incubus, a hideous goblin, like a depraved, priapic chimp, has climbed on to her chest. Are these merely dream-images, we have to ask ourselves, or has she truly fallen into the hands of these vile creatures of the night?
Fuseli's first audience may no longer have been totally credulous of folkloric, superstitious explanations of night terrors, sweats, sensations of falling and the like, but neither did these people yet have any of the benefits of modern sleep science. Girls were still advised not to sleep on their backs, just in case. In the daytime, though, they scoffed.
Even as The Nightmare was colonising the popular imagination, one critic called Fuseli (who was professor of painting at the Royal Academy) a "mad professor", and Hazlitt dubbed him "a nightmare on the breast of British art". For all that, his preposterous image wedged itself firmly into the popular culture, and thousands of people who have never heard of its creator will recognise it. Fuseli may, as it is thought, have cobbled his picture together using wholesale borrowings, but he undoubtedly created something that is more than the sum of its parts. It may be vulgar and rather ridiculous, but it is certainly unforgettable.
"Gothic Nightmares: Fuseli, Blake and the Romantic imagination" is at Tate Britain, London SW1 (020 7887 8008) from 15 February to 1 May