That makes Isthmus sound dramatic but not distinctive. After all, Rothko's paintings, James Turrell's installations, and even Anish Kapoor 's carved boulders do something similar - intensifying our consciousness of colour until we register it both as a sensual experience and as something divorced from the body. But the sensibility behind Isthmus is the antithesis of what Houshiary calls 'the Romantic vision of vagueness'. It is no accident that the exterior is so hard, the interior so shiny, or the gap so narrow - many viewers were too nervous to squeeze inside, and those who did were never allowed to forget the physicality of the work or, since they could see themselves reflected, of their own bodies. 'That's the nature of the experience: it's not an easy space to enter. Once you're inside, it's immense, but it doesn't allow you to become romantic. It is a sober experience.'
Houshiary is Iranian in origin, and so, too, is her understanding of the sublime. Her paintings take the form of transcription of Sufi chants - hundreds and hundreds of delicate, spidery calligraphic marks, so faint on their black background as to be, at first glance, almost invisible, but gradually welling up to create subtle webs and fluxes of form. Their power lies not so much in the specific texts (which Houshiary won't translate) but in this phenomenon of aesthetic form gradually manifesting itself in the apparently mindless business of incantatory repetition. 'It is not a cool, rational process, it's a discovery, a process of research. As soon as I say they are Sufi, people put them in a category, and think that they can't have access to them. But it's the process that matters, sitting there eight hours a day and doing the same thing. To reach that state of equilibrium requires intensity and discipline. That's why I don't use colour - anything but black and white would be a distraction.'
This kind of austerity and discipline extends to her decision to commute from Putney in affluent south-west London to a studio in Deptford in the south-east - set in the middle of some of London's most depressed housing. She admits that the unprepossessing environment may intensify her focused introspection: 'I read on the way there, and on the way back. I have my own space and, outside or inside, I stay in my own space.' The attraction certainly doesn't seem to be the company of the many distinguished artists working in the same block (Sean Scully, a near neighbour, was on last year's shortlist): 'My studio is quite isolated. I lock myself away for month after month. These paintings are difficult to do - they're satisfying because they're physically demanding. You can't make things like this by socialising.'
It's not that she's anti-social - she enjoys the team-work involved in having her sculptures fabricated from her drawings ('I work like an architect'). But her work is now so concentrated as to make outside influence irrelevant. 'I'm going so far from these things, the only thing that is left that is still powerful is poetry: the word itself is a breath, both physical and intellectual, a sacred thing. The rhythm of speech, of poetry is to do with number, which I'm fascinated by, and with music, which is absolutely central. I'm making harmony.'
That little rush of connections is crucial to Houshiary's art. Her abstraction does not grow out of the Modernist tradition, but out of the Islamic perception of the order of the natural world as an expression of the God-given, geometric inter-relationships of numbers - Mondrian's distillations of organic form would be the nearest Western equivalent. There is no room for Expressionistic improvisation. 'My sculptures are fabricated because they're not about me. Even the paintings are about repetition; they're filled with ideas, not with my hand print.' But nor do her works carry the explicitly anti-expressionist charge of the minimalist sculptures they superficially resemble. Yes, one piece of her three-part sculpture The Way to the Unseen consists simply of an upright cuboid shape but 'it is the exact opposite of minimalism, which was the denial of the transcendental; it aspired to become nothing, empty. If I make something empty, it is because it is a container of meaning.'
Where the conventional minimalist cube might have been seen as the end-product of a reduction towards purity, these look more like the beginnings of something. They might typically be open at the top, divided inside by a grid of vertical planes, creating upright rectangular spaces, some of which are plated with gold or silver foil, or highly reflective copper. Reflections of reflections open up ever-more complex spaces and structures. Viewers tend to circle sculpture at a respectful distance, but Houshiary's are characteristically to be seen peering deeper and deeper into an interior which, Tardis-like, seems far larger than the exterior should allow. Rather than cutting complexity away, she is dragging the outside inside.
That could stand as a motto for Houshiary's aesthetic ambitions, attempting as she does to express religious perceptions within an art world embarrassed by the very idea of transcendence. 'If I start talking about the culture I come from, or about this movement or that movement, people start narrowing me down into this or that pigeon-hole. I'm showing in a gallery in the 1990s, and it's a very narrow world. The world is much bigger than the language of art criticism can acknowledge.'
Even so, and despite her occasional impulse to throw it all in and reinvent herself as a poet, she remains convinced that the art world can itself widen the horizons of a culture which she sees as crippled by 'a poverty of imagination and language'. As a professional artist, she is relatively cool about her achievement in being shortlisted (she is, after all, already well-established, both here and abroad). But the prize is, she thinks, 'a good thing if it allows people to come to ideas - I don't mean my ideas, but in general. There is so much negativity here, but I do believe art can change things. I still think it is a very valuable thing.'
It is this combination of crisp intelligence and raw idealism that makes her such a welcome and, frankly, unexpected presence on the short list.
Houshiary's show at the Lisson Gallery, London NW1 (071-724 2739) ends tomorrow. The shortlist exhibition is at the Tate, London SW1 from 2 Nov-4 Dec
Next week: Adrian Searle meets Peter Doig
Photograph by Nicholas TurpinReuse content