One bright day, sometime around 1939, in a vacant lot amid the run-down, soot-streaked tenements of Harlem, four black children armed with makeshift sabers battled pirates on the imaginary galleon of a woodpile. About them the bounding main shimmered in the salty sunlight, the woodpile rocked gently with the swell, and the cries of querulous sea birds mingled with the ring of swordplay and the lusty bellows of the duelists.

Meanwhile, across the noisy, littered street, ragmen trudged aimlessly along the cracked sidewalks, unemployed young men hung out on corners in listless bands, and women bustled down to the fishmongers' and fruit sellers' stalls to try, with what little money they could scrape together, to buy enough food to feed their hungry families.

You can read many stories in the Depression-era photographs of Aaron Siskind, 60 of whose images are now on view at the National Museum of American Art. The prints were culled from several projects the photographer initiated before, during and after his involvement with the Photo League of New York, which he joined in 1936. Considered as a group, these pictures, many of which originally appeared in the 1981 book "Harlem Document: Photographs 1932-1940" and the never-completed project "The Most Crowded Block in the World," comprise an almost 20-year-long portrait of a section of what is perhaps America's most significant black neighborhood: the square block between 142nd and 143rd streets, and between Lenox and Seventh avenues.

Through these black-and-white "windows," we look behind the bandstands in strip joints, into the narrow kitchens and bedrooms of squalid apartments and along rows of shopfronts and boarded-up buildings. There is misery in many of the images, but there is tenacious vivacity too. There is poverty and filth, and there is nobility. But perhaps most importantly there is Siskind's unerring eye for pure composition, and his uncanny ability to capture with the click of a shutter a particular mood or character -- most importantly, because it's this talent that enables the artist to communicate the reality of his subjects, and allows us to empathize with them.

Down the road from NMAA at the Washington Project for the Arts, there is another exhibit that centers on New York's less privileged inhabitants. This multimedia installation by Krzysztof Wodiczko and a host of collaborators features photographs taken over the last two or three years, mostly of homeless people, mostly in and around Tompkins Square Park in the East Village. But these photographs, projected images, video and "Homeless Vehicle," a utilitarian device designed by Wodiczko and David Lurie, do not constitute a documentary in the sense that Siskind's pictures do. This show, "New York City Tableaux: Tompkins Square/The Homeless Vehicle Project," is not the work of a chronicler, but of an activist. Where Siskind recorded with compassion -- but distance -- the lives of everyday people, Wodiczko's approach is one of advocacy. This is not to say that Siskind didn't have a sociopolitical agenda. He professedly did. But he saw his mission as artist and documentor very differently. He believed that in presenting reality as perfectly as he could, he might move society to address that troubled reality.

Between these two shows lies the distance not just of time, but of artistic ethos and civic purpose. It would be hard to imagine a more succinct illustration of the changes wrought on 20th-century art during the 60 years that separate the substance and purpose of these very different artists' works.

The centerpiece of Wodiczko's show, first organized by EXIT ART of New York in September of last year, is the Homeless Vehicle, a combination pushcart, shelter and storage bin. Or, as described in the text that accompanies one section of the exhibit (and there is a lot of manifesto-like text, by Papo Colo, Neil Smith, Rosalyn Deutsche and Julie Courtney): " ... a strategy of survival for urban nomads -- evicts -- in the existing economy." By the very conception of this contraption, designed and built for actual use by homeless people, Wodiczko has abandoned his discreet position behind the camera, and determinedly placed himself in the picture. This artist, unlike Siskind, is not just recording a phenomenon, relying on his mastery of a medium to bring it alive, to present it for our consideration. He is actively influencing events and pointedly documenting that influence.

Where Siskind shows us a filthy tenement courtyard starkly defined in jagged shadows, a black Harlem nightclub stripper performing for a small group of whites, an Edward Hopperlike facade of a boarded-up building, portraits of a janitor and his wife, or a mother silhouetted in a doorway, watching her son play with a ball on the cramped living room floor, Wodiczko shows us organized rallies of the homeless at Tompkins Square, the same folks that he's armed with toy assault rifles, some of his own images projected on the sides of buildings, and, of course, various homeless people putting the Homeless Vehicle to use. Both his big, wall-sized projections and his color and black-and-white stills are snapshots, really, the sort of bald, purely functional frames you get on the TV evening news.

As with so much contemporary art, Wodiczko's imagery does not depend on aesthetics for its impact. It appeals only to one's sense of civic justice. It makes a political statement, pure and simple. The closest this artist comes to evoking the kind of poignancy everywhere evident in Siskind's tableaux is a color photo of a homeless person pushing one of the vehicles past the entrance to Trump Tower. And this, obviously, relies on rather heavy-handed juxtaposition to achieve its impact.

In contemplating these two shows, the viewer comes smack up against one of the more profound aesthetic issues of the age: At what point is the distinction to be made between form and content? Methodology and intention? Art object and political agenda? You must judge for yourself which are the more powerful: Siskind's compassionate but rigorously disciplined photographs of Harlem as he found it, or Wodiczko's equally compassionate but at best earnest recordings of a similarly destitute and downtrodden society as he would like to see it amended.

It almost begs the question: Which is the more deserving pursuit, art or activism?

Harlem, Photographs 1932-1940, by Aaron Siskind, at the National Museum of American Art, Gallery Place, Eighth and G streets NW, through March 17.

New York City Tableaux: Tompkins Square/The Homeless Vehicle Project, at the Washington Project for the Arts, 400 Seventh St. NW, through Jan. 20.