This talent for finding the surreal in the banal is one of the many ways in which Iturbide is influenced by Manuel Álvarez Bravo (1902-2002), her teacher and mentor. Drawing on the Mexican traditions that confront death, they both created densely poetic images. Look, for example, at Bravo’s photograph of a fallen sheet, made in the 1940s. By chance or by design, a white cloth rests on a tiled floor. This simple subject opens up a cascade of associations: the cloth looks like a shroud; its folds and bends appear to trace the contours of a human body; its placement on the ground makes you think of a corpse. This picture, an ancestor to the one Hernandez posted on Instagram, echoes another by Bravo, “Striking Worker, Assassinated” (1934), which shows a union leader lying in the street with a bloodied face moments after he was murdered. But what was raw photojournalistic reportage in the earlier picture is transformed into a different kind of strength in Bravo’s photograph of the fallen sheet. The dead man is an instance of death, but the sheet on the floor becomes Death itself.

Surrealism of this kind often relies for its effect on humanity in the absence of actual humans. In a 1929 photograph by Eli Lotar, two rows of calves’ feet outside an abattoir imply a butcher, who is nowhere to be seen. But the feet also make us think of our own feet and, horrifically, of amputation. They have a stance, an attitude, just as Bravo’s fallen sheet has an attitude. Attitude also suffuses the trees in a long-term project of Iturbide’s, in which she photographs “plants in therapy.” Taken in Mexico, Italy, Japan, Mozambique and elsewhere, these photographs are far from ordinary botany or taxonomy. One image from Rome, for instance, features tall plants covered at the top with a black cloth. Are they executioners? Or the condemned?

Certain objects tend to recur in surreal photographs: shirts, bedding, ladders, chairs, shoes. Designed to accommodate our bodies, altered by their encounters with us, they retain something of that humanity even when they are not being used. Forms that mimic the human are frequent, too: animals, trees, silhouettes, prostheses, wigs, mannequins, puppets. And there are many surreal photographs that feature actual humans, humans whose bodies (like those of Iturbide’s Holy Thursday siblings) stray into otherness. But there’s no guarantee that any given photograph will be surreal. Most photographs are not interesting. Then a strange one turns up, a real winner, and it is difficult to pin its strangeness down.

The surreal image caught on the fly can remind us of our vulnerability much more powerfully than manipulated photographs can. A double exposure that gives a man two heads is too definite, like something from a horror film. Its heavy-handed “surrealism” robs it of pathos. But a protuberance on the trunk of a tree, or a chair with a missing leg, or a twisted metal railing in the midday sun, being more ambiguous, can be more surreal. A main feature of surreal images is that they invite active verbs: Things pour and shimmer; they push and spray and brood, as though they had intentions. These kinds of images, in which the inanimate is suddenly animate, generate an open-ended visual conspiracy.

The photographic surreal, like the sublime or the obscene, is subjective. It cannot be locked down to a theory, codified and filed away under an “ism.” Rather, it arrives like a metaphysical gift, showing up when it is least expected to conquer logic and haunt the imagination.