I KNOW IT when I see it," wrote former Supreme Court Justice Potter Stewart of obscenity. The same applies to "Folk Art," for "folk art," like pornography, is easier to recognize than it is to define.

What qualifies as "folk art"? That old elusive question nags the "Folk Art" exhibition now at Middendorf/Lane Downtown, 404 Seventh St. NW. What is that strange something, that eerie bond of sympathy, that holds this show together? There are seven artists in it. What is it they share?

Surely not materials. Ned Cartledge whittles wood, Lanier Meaders models clay, and Newland Sparks prefers to work with Coke bottles and flashlights, plastic tubes and toys. These artists share no image. While Nellie Mae Rowe paints her colored dreams of paradise (flowers in the garden, fish swimming in the trees), R.A. Miller dresses Georgia hills with windmills by the hundreds (that spin, but do not mill). Nor have these artists freed themselves, as is so often claimed, from the constraints of tradition. Eighty-one-year-old Carlton Elonzo Garrett, who whittles little people and little farmyard animals, is not the first to do so. Georgia potter Lanier Meaders, although he is not black, makes merry-yet-maniacal "devil jugs" that are direct descendants of pots thrown in West Africa 500 years ago.

The artists represented here come from Buckhead, Flowery Branch, Saint Matthews, Spruce Pine and other rural southern towns with names just as melodious. Many seem to be entirely self-taught. We recognize at once what their "folk art" isn't.It isn't "high" art or "official" art or art that's tuned in any way to chic esthetic theory. Nor was it proing and amusing political cartoons. Dan Miller's art, in contrast, though as heartfelt and as powerful, is the opposite of topical. His carvings seem to represent forces he alone has seen, guardians known to no one else, icons that have somehow climbed out of his soul.

That sense of spirits speaking haunts this vivid show. The finest artists here are mediums of a sort. We do not doubt their motives. Embedded in their southern souls, somewhere in between revelation and obsession, time-killing and toy-making, is a shared, yet private realm from which these semimagical, friendly objects flow. What makes "folk art" good is its psychic authenticity. This show was chosen by George Hemphill, who, like the artists in it, did his work with wonder and affection. It will remain on view Wednesdays through Saturdays, noon until 5 p.m., through Jan. 7. Richard Depsey's Show

Richard Depsey's "miniretrospective" at Nyangoma's Gallery, 2335 18th St. NW, is not an easy show to like, Dempsey has been working here since the 1940s, and his unswerving loyalties -- to medernism's mainstream, to Haiti and Jamaica, and the black community -- are apparent everywhere. But he never soothes the viewer. There is growling in his art. His "Cave Queen Ceremony in Jamaica" is a work that frightens; equally unsettling are the gray bones shining through gray flesh in his surreal, spider-ridden paintings from the '50s. His later abstract expressionist pictures are even more disturbing: He never strokes the paper, he hacks at it and stains it. In his "Metamorphosis -- Black Sunburst," the paint is thickly clotted, his brushwork suggests violence, his colors evoke gloom. Even in the most pastoral of his pictures, for example, his dry-brush drawing "Market Day, Barranquilla," one hears a kind of pounding, of brush beating on paper. Viewers will not smile, but will shudder at this show, which closes Dec. 12. Schroeder Cherry

The Noa Gallery, 132 Rhode Island Ave. NW., is showing the innocent and charming pleasure-giving paintings of Washington's Schroeder Cherry. Looking at his art is like going on vacation. His images -- sand castles and bathers, cool rum drinks and drowsing orange cats and sailboats that glide on bright blue sun-warmed seas -- smile at the viewer. Schroeder's flat and friezelike figures -- his healthy, playing children, his luxuriating ladies -- seem to be assembled from nostalgic memories of sunny tropic days. His colors are delicious. Schroeder's paint is layered, so that what seems to be one color is many; beiges, browns and other subtle earth tones twinkle through his blues. He must be a happy guy. His show closes Dec. 22.