Salvator Rosa at Dulwich Picture Gallery

The exhibition of Salvator Rosa (1615-73) at Dulwich Picture Gallery more or less coincides with the Turner prize, and what wouldn't we give for an artist cut from similar scintillating cloth in these dreary artistic days? The Naples-born painter, poet, musician, actor, satirist and wit was the first major visual artist to be an outspoken social critic and diehard alienated outsider. In this respect he makes Caravaggio, the shooting star of the previous generation, look inarticulate.

Not only did Rosa specialise in edgy new subject matter – portraits of disgusted philosophers and disdainful hermits; lurid twilit scenes of fiendish witches and brutal bandits in apocalyptic wildernesses – he was also a daring self-publicist. Rosa showcased his most ambitious and sensational paintings in some of the first ever temporary exhibitions, at the Pantheon in Rome, all the while ridiculing the patron classes and modern mores in scathing satirical poetry and theatrical comedies: "Whenever he moves or speaks," marvelled a friend, "he dislocates the audience's jaws." Rosa's proto-romantic credo is tersely expressed in one of several self-portraits, smuggled into the side of a tumultuous battle scene painted for the Grand Duke of Tuscany. The elegantly moustachioed, pale-skinned Rosa crouches behind a dying horse. He dispassionately observes a Christian soldier sliding his sword blade into the blood-soaked throat of a fallen Turk whose right forearm has just been hacked off. The artist insouciantly holds up a silver shield inscribed with the first two letters of his first name and surname: SARO. This is Italian for "I will be".

Salvator Rosa was born into a family of Neapolitan painters and builders in 1615, but his surveyor father died after a long illness in 1621, leaving the family in poverty. The Spanish-ruled port city was the second largest in Europe after Paris, with a population of between 300,000 and 400,000, many of whom were homeless and only intermittently employed. The population was swelled by migrants from the countryside, where the taxes imposed by the Spanish colonial masters and the local aristocracy were even more onerous than in the city. Spanish involvement in the thirty years' war, starting in 1618, was financed by further taxation of the poor, leading to uprisings in Naples in 1620-22 and 1647-8 (the latter prompted by a new tax on fruit). Rosa's anti-establishment tendencies, and loathing of the Spanish, were fostered in this exploitative environment: in his satires, he bewailed the condition of "my country, slave of slaves"; railed against hypocrites who bought pictures of beggars while ignoring real beggars in the street; and praised Masaniello, leader of the 1647 uprising, "the low-born fisherman, barefoot, a worm".

Rosa and his elder brother Giuseppe were fortunate to be educated in a new free school for the children of the poor run by the enlightened Piarist order, who welcomed the ideas of Galileo and the "new science". Both were outstanding students, especially of painting and literature, and for the rest of his life Rosa was most at home with the cultural and intellectual elite. Painting in Naples operated in the long lugubrious shadow of Caravaggio, who had worked in Naples and Sicily in the four years before his death in 1610. Rosa was probably trained by the Spanish follower of Caravaggio, Jusepe de Ribera, brilliant exponent of a delirious form of dirty religious realism, and by Aniello Falcone, a specialist in a new kind of densely crowded, frieze-like battle picture without an obvious hero.

Falcone also did still lifes and landscapes. He and Rosa drew and painted in the countryside around Naples and along the coast, populating their pictures with low-life. "Painting from nature" was a major innovation of the time, pioneered by Dutch and Flemish artists. Rosa struggled to make a living, however, and in 1635 he moved to more promising pastures in Rome, undisputed centre of the art world. But in 1640, having publicly lampooned a theatre company run by the dominant artistic figure, Gianlorenzo Bernini, he moved to Florence where he embarked on a passionate, lifelong affair with a beautiful married woman, Lucrezia Paolina, whose husband had permanently left the city. He stayed for 10 years until, incensed by the corruption of court life that he attacked in his first satires, he returned to Rome.

As a landscape painter, Rosa came to be seen as the sublime antithesis to the beautiful pastoral idylls of the Rome-based French painter Claude Lorrain (1600-82). In the 18th century, he was virtually treated as an honorary Englishman and the proto-romantic painter. English milordi bought up nearly all his work. In 1717, Horace Walpole famously saw the Swiss Alps through Rosa-tinted spectacles: "Precipices, mountains, wolves, torrents, rumblings – Salvator Rosa". Yet there is nothing solidly or crisply alpine about Rosa's world. Rather, he is the poet of precariousness, revelling in the fragility of giant rock formations, trees, buildings and ships. At any moment you feel his tiny figures could be swept away or snapped in half like flotsam and jetsam. His great trees always have several branches that are inexplicably broken. The splintered stumps render them far more forlorn than unrestored antique sculptures, and they can even seem like a premonition of Paul Nash's bomb-blasted trees from the first world war. The famed fertility of Naples' hinterland is conspicuous by its absence.

Rosa's largely tragic feeling for landscape must have been informed by geological as well as by historical realities. The greatest eruption of Mount Vesuvius since antiquity, and the first since 1500, took place in 1631, accompanied by earthquakes. Vesuvius's cone collapsed, with molten lava pouring into the sea and ash reaching Constantinople. Three thousand people died amid general devastation. There's a strong sense of what one could call "seismic shiftiness" in Rosa's landscapes. There are few clearly demarcated ground lines, horizon lines, or perspectival vanishing points. We (metaphorically) stumble and grope our way across his essentially pathless landscapes; the ground regularly falls away into gaping gloomy hollows and swampy expanses that have a hypnotic gravitational pull. Sometimes, this feeling of porous, uncharted territory is liberating, akin to that famous Situationist slogan: "Beneath the paving stones – the beach!"; but mostly, it's chastening, like seeing the skull beneath the skin.

The 1631 eruption certainly stimulated interest in volcanoes. The Jesuit scientist, Athanasius Kircher, visited the seismic zones of southern Italy in 1637-8, seeing Etna from afar and peering into the smoking crater of Vesuvius. In the 1660s, by which time Rosa was exploring highly esoteric subject matter, he painted the spectacular death of the pre-Socratic philosopher Empedocles, who had fallen into the mouth of Etna. In antiquity, various explanations were offered. Empedocles had jumped in to prove he had become a god; or to gain fame; or, more bathetically, he had slipped. But during the 17th century, he was treated as a martyr for the cause of scientific research. Empedocles taught that the world was composed of four elements, earth, air, fire and water, and his heroic death came about from his fearless determination to study volcanoes at close hand.

Rosa's picture is one of the great evocations of the reckless pursuit of knowledge and power. Empedocles sky-dives through the air, already engulfed by jagged agglomerations of dark tufa rock and plumes of smoke. His red, blue and yellow robes are spread-eagled epically; his massive arms and hands are at full stretch. Yet he is going down to his death. Rosa has shown his bronze slipper perching on a rock ledge: in antiquity, this was said to have been thrown up by the volcano, thus proving he had perished and was not immortal. Empedocles is very much a fallen angel – and his exact catholic antithesis can be seen in the permanent collection at Dulwich, in Poussin's Translation of Saint Rita of Cascia (1630s), where the good lady levitates up to heaven on a sparkling sunlit cloudbank, arms spread aloft.

Rosa's Empedocles makes one think of the passage about the fall of Satan at the start of Milton's exactly contemporaneous "Paradise Lost" (1667):

Him the Almighty Power

Hurled headlong flaming from th'Ethereal Sky

With hideous ruin and combustion down

To bottomless perdition, there to dwell

In Adamantine chains and penal fire

Yet Rosa marvels more than he mocks: he has made Empedocles a splendid dead ringer for the creating God on Michelangelo's Sistine ceiling.

Rosa's attitude to his human subject matter, whether they be bandits, witches, hermits or fishermen, is rarely clear. The description of Masaniello – "the low-born fisherman, barefoot, a worm" – is a case in point. The sympathy of the words "low-born" and "barefoot" is not carried through into "worm", which puts a satirical sting in the tail. The primary meaning of the "worm" reference is that this is how the Spanish overlords designate the average Neapolitan (King Lear's "forked animal"). But the secondary meaning is that Rosa also finds them and their destitute state contemptible – he despises the fact that he and his fellow countrymen have allowed themselves to become a "slave of slaves". These evaluative changes of gear are typical. The shiftiness that pervades his work is psychological and moral as well as geological – and therein derives its nervous energy.

Bandits were a perennial problem in the countryside, terrorising rich travellers and poor peasants alike, but they still became a popular subject for art in the 17th century. Some were regarded as noble savages or Robin Hoods, romanticised for their libertarian lives, sense of natural justice, and their aesthetic sensibilities: the legendary bandit Marco Sciarra had reputedly fallen to his knees when he met the writer Torquato Tasso on the road, and had let him pass unharmed. Rosa's bandits range from being thuggish assassins to the dreamy swains found in Venetian pastoral paintings. They are invariably glamorously dressed. In the 18th century, a myth arose that Rosa had been in a mythical band of soldier artists who had taken part in Masaniello's revolt. They were no doubt influenced by Rosa's darkly swaggering self-portrait in the guise of his stage character Pascariello, a supremely sexy sword thrusting from his hip.

Rosa's witchcraft scenes are only rivalled by Goya, whom he influenced. In around the 1630s the witch-hunts reached a climax, but they went into rapid decline with the end of the wars of religion that had first been set in motion by the reformation. The rational scepticism made fashionable by the scientific revolution also played a part. Ostensibly, collectors would meditate on a scene of witchcraft in the same way they would meditate on a scene of hell, or on a memento mori (a self-portrait from 1647 shows a tearful Rosa writing on a skull). But Rosa's aristocratic patrons were also fascinated by magic, astrology and alchemy, and gathered "marvels" together into cabinets of curiosities. The distinctions between superstition, religion and science were very blurred.

Helen Langdon, author of much of the hugely informative catalogue, believes that Rosa's largest witchcraft scene, painted at the same time as the skull self-portrait, is a satire of witchcraft and of all those who believed in it. The way in which the hideous naked witches and their creepy accomplices are arranged in a foreground frieze certainly is theatrical. But it's far from being mere pantomime. There is a hallucinatory intensity to the individual scenes – the "fumigation" of the hanged man; the presentation of the sacrificial baby; the skewering of the heart; the skeleton being helped to write with a quill pen; the placing of the voodoo doll before the mirror. These are highly trained professionals, a hag's army going about their magical business. We can't help but be mesmerised by their meticulousness and artistry. Rosa implicates himself and his own "diabolical" artistry by inscribing his signature with a calligraphic flourish on a rock sat on by the fleshy arse of the witch holding the voodoo doll. This isn't such an indignity as she's by far the youngest witch, with a lovely body – possibly modelled on that of Lucrezia. Make no mistake: this is Parnassus as well as Golgotha.

Some modern critics find all this sinister stuff preposterous – as did Rosa himself in later life when he renounced genre scenes depicting rogues in favour of "learned" philosophical art. In Landscape into Art (1949), Kenneth Clark drily dismissed the English Rosa cult of the 18th and early 19th centuries: "As Charles II said of a popular preacher, 'His nonsense suited their nonsense.'" John Ruskin was surely a better judge. In Modern Painters, the first history of European landscape painting, he made an astonishing claim: "I see in him, notwithstanding all his baseness, the last traces of spiritual life in the art of Europe . . . All succeeding [painters], however powerful, would have mocked at the idea of a spirit. They were men of the world; they are never in earnest, and they are never appalled. But Salvator was capable of pensiveness, of faith, and of fear." This long overdue exhibition (the first for nearly 40 years) will bring all that thought, faith, fear – and fearlessness – back into view.

Salvator Rosa: Bandits, Wilderness and Magic is at Dulwich Picture Gallery from 15 September to 28 November. For details, go to dulwichpicturegallery.org.uk