Ron Arad’s steel D-Sofa (prototype, 1994)

I’m standing in New York’s Museum of Modern Art, contemplating a large metallic blob graced with a rear end-sized dimple, and I have the urge to sit on it. But I can’t see how to match my curves to its patinated superplastic aluminium hollows. Do I perch? Can I recline, curl up or drape myself across it? No clue. In any case, the question is answered by a defensive line of labels warning me not to touch it at all. This, after all, is MoMA.

The chair (if that’s what it is) forms part of No Discipline, a retrospective of Israeli-born, London-based sculptor/designer Ron Arad. Navigating the show is nearly as befuddling as sorting out how to put its contents to use. His objects – some conceivably functional, others frankly forbidding – are displayed in a great steel grid of shelves that leans, folds back on itself and, with a slightly menacing glint, challenges viewers to enter. Once inside, I felt as if I had walked into a 3-D chart, a periodic table of design elements, perhaps.

Within the body of Arad’s work, each piece belongs to its own kingdom or family. How these categories relate to each other is tough to tease out and doesn’t quite seem worth the effort. To help the viewer through the rigid organisation of No Discipline, curator Paola Antonelli has provided a double-sided, large-format handout, an unwieldy guide to the grid, showing schematic sketches of the shelved objects. But even equipped with map and audio guide, I found myself wandering in and around the torqued space, asking guards for directions.

Arad enjoys a degree of confusion. The show’s title refers both to his intuitive working methods and to his refusal to recognise any distinction between sculpture and design. Educated first at Jerusalem’s Bezalel Academy, and later at the Architectural Association in London, Arad found himself ill-suited to the restrictions of conventional employment. He wanted the freedom to move between professions and to make his studio a place where fantasy could rattle around unmoored.

Arad’s Chalk Farm Road Studio (in video, 1989-91)

“I like to have fun,” he says in an interview published in the show’s catalogue. “My office works like a kindergarten – which is my way of not having to go to work, so you don’t start the week groaning that it’s Monday. And that’s the basis of everything, this playground.”

His style combines various strains of surrealism. Echoes of Meret Oppenheim’s fur teacup resonate in Arad’s impossible “Bouncing Vases”, made of plastic coils that might hold a single rose but not a thimbleful of water.

Arad made his debut with “Rover”, an armchair that evokes Picasso’s way of playfully welding found objects into organic forms. Back in 1981, he scavenged a pair of trashed red leather car seats from a Rover , hitching them to tubular steel bases with Kee Klamp joints. Displayed in the window of the so-far profitless studio, these masterpieces of bricolage caught the passing eye of fashion designer Jean-Paul Gaultier, who stormed the closed shop and bought both. Arad’s reputation took off.

“Rover”, made until the supply of usable seats was exhausted, evolved 26 years later into “Moreover”, a set of Vitra limited editions in a choice of rusted steel or chrome. But what’s missing in the next generation is the handmade honesty, the makeshift funkiness of the original.

Arad retraced that transition from rough to sleek many times in his career. Take “The Volumes”, hollow armchairs assembled from scraps of tempered steel. The first, “Big Easy” (1988), flaunted its welded seams and took pride in its austerity; it luxuriated only in irony: a cartoonish lounger that defied human comfort. Later iterations become more streamlined and user-friendly. The engulfing, red “Soft Big Easy” (1990) is a distant descendent of the Spartan original .

If Arad loses his way when his work becomes too comfortable, he also tends toward the generic when he veers into straight-up sculpture. The bulbous humanoid globules of the “Bodyguard” series look deluxe, labour-intensive and dour. They are definitely not made in a kindergarten.

MoMa’s “Do Not Touch” signs are clues to the ambivalence in Arad’s work. Fashioned out of gleaming industrial materials, his furniture only looks mass-produced; in fact it’s craftsmanly, rare and precious. (During one of the art market’s boomtime dizzy spells, in 2006, one of his sofas sold at auction in New York for $409,000.) In the very few homes that have an autograph Arad in the living room, I imagine guests, like MoMA’s visitors, trying to puzzle out how they might, theoretically, sit in it.

‘Ron Arad: No Discipline’, MoMA, New York, until October 19. www.moma.org