Out of this world: The Museum of Everything

You don't have to be mad, but it helps. When it comes to Outsider Art, that's a good general motto, but nobody has a definition. For example, was William Blake an Outsider? You could make a case: the symmetrical images packed with detail, the visions, the home-made mythology. Yet Blake studied at the RA, knew the European tradition, consorted with major artists of his time, sought an audience. In those ways, his was the very opposite of Outsider Art.

In Britain we don't see enough of it to judge. Tate Modern doesn't collect it, though it's clearly a manifestation of modern art. It turns up occasionally – there was a show at Pallant House in Chichester earlier this year. But now there's something more comprehensive. The Museum of Everything (as it's called) has just opened, showing nothing but a large spread of Outsider art, from the collection of James Brett.

To British eyes, Outsider Art has meant Madge Gill, Scottie Wilson, Louis Wain, maybe Alfred Wallis (or is he Naïve Art?) Madge Gill is certainly wonderful, her ghostly faces hovering in densely woven pen-work. But her obsessional, mediumistic procedure is only one Outsider style.

Outsiders can be neat or spontaneous. They make repetitive miniscule patterns and signboards with bold painted messages, elaborate gadgets and creepy puppets. They deal in dream creatures, sex fantasies, secret societies, ideal cities, religious and political propaganda. With 90 artists from Europe, America and Asia, The Museum of Everything has the range.

It's situated at the foot of Primrose Hill, in a very chi-chi corner of North London. But take a turning off a side street, and the tone changes. At the end of an alley, it's like the entrance of a crooked house in an old fairground, with folksy lettering and a rickety booth.

Once inside, things change again. You're in a large building, evidently in mid-conversion, half-stripped and deconstructed, a warren of offices and corridors with knocked-through walls, naked iron steps, a huge warehouse extension. It was a dairy, then a recording studio. Now it's an exhibition space, a consciously non-gallery, appropriate to the non-standard artworks in it. Well, that's the idea.

Maybe the mood is too "fun" and "crazy". (Some Outsiders are rigorously fastidious.) But the use of the chaotic space is effective. Rooms are made into effigy-theatres, to be viewed through the doorway. Cramped corridors imitate cramped mental states. There are surprises when you go round a corner, and a vast wall is hung high with fantastical paintings (binoculars provided).

This is old-school showmanship. You might almost call it a freak show. It's not particularly respectful of its artists. They're not presented as the heroes of "raw vision", nor the saints of creative therapy. But if you're going to display this art, you might as well go all the way; bring out the power of its weirdness with a full staging. Especially since this Museum isn't really a Museum. It's a brief splash in a temporarily vacant space. Get along before Christmas.

And for no other reason, go to see the works of Henry Darger. The Chicago janitor recluse now stands as the genius of Outsiderism. His vision was so complete, his artistry so inventive. The Museum has about a dozen of his panoramic scenes, crowded with figures that Darger traced from various sources – comics, magazines, children's colouring-books. They generally show a multitude of prepubescent schoolgirls – the "Vivian Girls" – suffering some persecution. Often they have male genitals. Extremely violent, bizarre and religious imagery abounds.

If Darger's perversity wasn't so eccentric, his work might attract the interest of the police. (Morton Bartlett's highly realistic dolls, mesmerising as they are, are another matter.) And if Darger he wasn't so visibly deranged and damaged, he'd be one of the greatest Surrealists. His collage techniques make Max Ernst's Une Semaine de Bonté look pitifully tame and artificial. What's so strange about Darger, of course, is that he isn't trying to be. He's trying to tell his story.

This is what makes Outsider artists so fascinating. They don't know what they're doing. Their work is not just strange, intense, original: it's compelled. However neat its making, it is essentially out of control.

Look at Emery Blagdon's complex and superbly crafted wire mobiles, hanging in a cupboard space. They're part of his Healing Machine, a gigantic contraption designed to give this Nebraska weirdo immortality. Or look at this beautiful, graceful-awkward abstract painting, massed yellow arrows moving in a curling formation. Its maker Forrest Bess, hermaphrodite and hermit, created these mystical signs for magical purposes.

Keep looking. Karl Josef Rädler seems to be a very innovative illustrator. His pictures typically show a half-length figure, staring at you; meanwhile, behind or around it, some action. He seems to have invented a genre, the "narrative portrait", or perhaps a new kind of picture storybook. But Rädler was confined in a sanatorium. These images all record his daily experiences. And then there's Judith Scott. She made sculpture from household objects entirely hidden by being wound-about over and over by wool and yarn. They're very convincing works. Scott had Down's syndrome, and only communicated through these things.

Both Bess and Rädler I would like to see in a one-person show. But with all these artists, and many others here, there's the same double-take. Art or mad? Why not admit that this work is simply art, and never mind who made it? Or why not admit the opposite: that our delight in this work is inextricable from our sense of its mental wildness?

There are three charming little outline drawings by Heinrich Reisenbauer, an artist from the Psychiatric Hospital at Gugging outside Vienna. They're very orderly. They show identical things arranged in regular rows and columns. One has cups and saucers, another bare trees, another umbrellas. And if you see these pictures as Outsider Art, you probably see them as a case of obsessive-compulsive neatness, the desire to organise, expressed in drawing.

But if you came across these pictures in a contemporary drawing exhibition (they'd fit in fine) you'd see them differently. You'd look for purposes. You'd become aware of a tension between order and the human hand. All the 60 cups and saucers are trying to be the same cup and saucer, but they can never be absolutely identical, because the drawing hand is not a machine. However firm it is, tiny little differences will enter into the repetitions, and these are to be noticed. That's what (among other things) the drawing would be "about".

And what would you rather these pictures were? You can enjoy them in two ways. One, you're enjoying somebody's madness or eccentricity, a spectator to their private and involuntary activity. The other, you're enjoying the playfulness of a self-conscious artist, something you can take part in. But you're looking at exactly the same picture. It works perfectly with both responses. Reisenbauer passes as artist and Outsider artist. Better mad or sane?

That's an extreme case. Mostly there is some hint, more than a hint, that an Outsider has been at work: some obvious excess or unbalance or craziness. Whatever, it comes down to is a feeling of solipsism. Outsiders are really deep Insiders. Their work is made in a sealed world. And what brings it into its extraordinary existence is also what makes it essentially uncommunicative.

The Museum of Everything is a great show. And let's hope it turns up somewhere else, because this is an art you can never get your head round. You can wonder at it. You can diagnose it. As an artist, you can learn from it. But you can't make company with it. It never speaks to you, only to itself.

The Museum of Everything: Sharpleshall Street, London NW1; until Christmas; every day; admission free ( www.museumofeverything.com )