IS MINIMAL DIFFERENCE a major subject? Even so-called identicals, Marcel Duchamp observed, reveal shades of difference, which he memorably called infra-thin. Roni Horn’s work mines that proposition exhaustively, asking us to find differences in things and then to question why we need them. For Horn, the terrain of likeness—that which is similar, the same yet different, akin, or simply close—encompasses a panorama of experience. Her work dwells on differences of degree that, she suggests, constitute as profound a representational problem as the starker discrepancies that conventionally give us our social and sexual selves.

“Roni Horn aka Roni Horn” at Tate Modern offered an expansive and eloquent presentation of the artist’s corpus of drawing, sculpture, photography, and books. Co-organized with the Whitney Museum of American Art and curated by Donna de Salvo, Carter Foster, and Mark Godfrey, the show concatenated bodies of work that are individually concerned with similarity yet in combination yield an insistently heterogeneous oeuvre. As such, it adroitly staged Horn’s simultaneous preoccupation with minimal difference at the level of the series and maximal discontinuity among modes of making. Horn’s art suffers from more selective presentations, where it can seem obscure or diffuse rather than concentrated. In this setting, it came into its own, in part because the conditions, even the geographic and meteorological ones, dramatized the work’s sustained exploration of the effects of light and water.

The museum’s position on the banks of the Thames, and its high windows offering dramatic views northward over the river to St. Paul’s Cathedral, provided an unparalleled context for Still Water (The River Thames, for Example), 1999, a suite of lithographs based on photographs made from different vantage points in the city. The series benefited from being installed in the riverside galleries, directly overlooking the source, a curatorial move echoing the artist’s own incitement of viewers to compare like with ostensible like. Almost any route to Tate Modern crosses the Thames or snakes along it, bringing the prospective visitor into range of the fast, murky, dangerous river that, as the artist has observed, has accrued an air of menace from its attraction of suicides. Each closely cropped image of Still Water delivers a straight-on, or downward-facing, shot of the water’s surface. Frozen by the camera, eddies take on the granular appearance of sulfurous sand, the density of molten metal, the brittleness of rock, or, as Horn suggests in footnotes to the images, the gelatinous consistency of aspic. The flecks of foam and flotsam that bob along the surface of what the artist calls “this substance,” Thames water, are echoed in minuscule white numbers printed on the images and keyed to dense footnotes below. These encompass a stream, or sludge, of literary references, diaristic reflections, and true-crime stories delivered in a deadpan worthy of a Dickensian detective. Upon hitting the cold water, murder victims, it seems, are liable to drop to the river bottom and sink into the mire. Corpses disappear forever or surface in parts.

Horn’s work is not exactly morbid, but it carries an undercurrent of abjection. “You have to admit, you can’t really look at the river in some places without thinking of shit,” asserts one of many rogue footnotes, which eschew the didactic in favor of the irreverent. Still Water’s presentation drolly induced viewers to ponder this proposition while watching the swirling currents race along below, as boatloads of tourists glided by on pleasure craft (among them Tate’s own shuttle boat). Referring to the tiny white numbers, in one footnote, as “litter,” comparable to “rubbish floating in the water,” the artist assigns herself the role of double agent, posing a hazard to the kind of romanticism her work also openly courts. Pink Tons, 2008, a frosty, quartzlike cast-glass cube that reposed in iridescent splendor in the Still Water room, displayed that other side of the artist’s attraction to the elements, to the mercurial life of “substances,” to weather.

In the British Isles, the appreciation of weather is so refined that meteorological reports are not only prospective but retrospective, and as Olafur Eliasson’s wildly popular Weather Project in Tate Modern’s Turbine Hall demonstrated, weather work has not lost its fascination since the time of Turner. Still Water charts the vicissitudes of weather effects on the river, from rain-spattered, with countless tiny vortices etching a delicate script into the water’s surface, to sun-dappled, tracing the currents with a thick calligraphy of light. These effects appear in real time outside the gallery, but also inside, observed in the play of light on the cast-glass works—glass being, as Ann Temkin observes in the catalogue, a substance with a viscous history. It is “a supercooled liquid,” she writes, one that “appears still but carries the vivid memory of its fluid state.”

Memory—in particular what might be called passing memory, such as the memory of yesterday’s weather—is a cardinal theme of Horn’s art and is in part what makes the work seem, as Briony Fer aptly describes it in her catalogue essay, “tenuous.” Quotidian memory falters or is difficult to grasp. One might say it has a tendency to be infra-thin. Take Things That Happen Again: For Two Rooms, 1986: First, the viewer enters a room in which a truncated copper cone rests on the floor, the cone here surrounded by a suite of drawings. In an adjoining room, an identical cone is installed in company with a series of similar, but not identical, sheets, arranged somewhat differently from the previous set. The ensuing double take slyly reminds us that memory, our fragile mastery over the past, is vulnerable even to the brief passage between rooms.

Dividing her time between Manhattan and Iceland, the artist has long designated the latter, volcanic island (and not the heaving metropolis) as the imaginative locus of her meditations on the speed of change and the fragility of memory. Miming Monet at Giverny—taking the measure of the weather day after day as the landscape is inexorably altered by industrial development and environmental depredation—Horn has created her own private Iceland. You Are the Weather, 1994–95, fills a room with portraits, arrayed in sets, of the same young woman pictured in close-up, staring directly at the camera, as she surfaces from a pool into steam or fog or blazing sunshine. Treated as a serial motif, the portrait marks time in the infinitesimal variations of a face from day to day, the installation an almost Nabokovian diary of a journey across Iceland that seems not so much erotically charged as erotically staged.

Horn’s artistic origin myth, at once erotically invested and flatly disavowed, begins in Iceland, as attested by a set of small graphite and watercolor drawings made on its coast in 1982. In the later drawings that formed the core of the show—and provided its most compelling interest—the artist’s mythology is trumped by what Fer suggests is a form of natural history. Using dry pigment suspended in varnish to render complex surfaces and faceted shapes or to produce radiant symbols of a gemlike intensity, Horn invites reflection on the mineralogical sources of her art, on what is literally mined for its production. In certain drawings, objects resemble geologic or entomological specimens, like the wings of an insect sectioned by a blade. The sheets themselves are scored and smudged, as if to underline the affinity between the work surface of drawing and kindred pursuits entailing the close examination of samples and their seemingly minute differences. A pair or trio are set out for comparison like cups on a table or splayed like halves in the process of being separated or sutured.

Number and series are concerns of Horn’s work in a somewhat different way than they were for the Minimalist project. Here, number is social: It designates the couple, the threesome, the group. More recent drawings summon a larger social nexus, its lateral sprawl, its crowding in and thinning out, its uneven distribution and relentless fragmentation. Intricate in facture, composed of sheets cannibalized from previous drawings and laboriously recombined, they chart the process by which something once continuous, such as a length of string, is segmented and then pieced together like some convoluted tale being recalled to memory—not as a seamless sequence of events but in broken lines, accompanied by a singsong litany of automatist rhymes, always only faintly penciled onto the vast surface of the page like static on the line: SHOW JOE / SO LOW / NO POE.

Or, then again, PEAR / PAIR. The Tate show fittingly opened with two works from a series of propped aluminum poles overlaid with lines from Emily Dickinson, including one in which the homophone “pear/pair,” quoted from one of the poet’s letters, announces Horn’s shared affinity for the doubleness of things. That deft introduction to the structural ambiguity of Horn’s work, however, was at times undercut by an extensive reliance on the artist’s words for the show’s text panels and the catalogue, where a lavish suite of paired photographs of the artist also appeared. This was testimony, perhaps, to the persistence of a biographical imperative in monographic shows—even where, as here, the question of identity is articulated as the very problem of being supposedly identical with oneself. Something was lost in this maneuver, even if it was archly performed as an act of self-splitting, right down to the two-volume catalogue. This is a body of work that thrives on absence, even when alluding to its author, and the absence of the artist was one I missed.

“Roni Horn aka Roni Horn” is currently on view at the Collection Lambert, Avignon, France, until October 4. The exhibition travels to the Whitney Museum of American Art, New York, Nov. 6, 2009–Jan. 24, 2010; Institute of Contemporary Art, Boston, Feb. 19–May 9, 2010.

Mignon Nixon is a professor of art history at the Courtauld Institute of Art, London.