The Iris and B. Gerald Cantor Roof Garden, at the rear of the Metropolitan Museum of Art, is nestled just above the trees and set into the park a good, quiet distance from the streets. Unmoored from Fifth Avenue, it offers an island or ship’s-eye view of the city. Before September 11, I often thought of the Met’s roof garden as the deck of Manhattan and of the World Trade Center as its crow’s nest.

Since the Cantor Roof Garden opened as an exhibition space in 1987, the Met has mounted annual shows there of modern and contemporary sculpture by artists such as Ellsworth Kelly (1998), David Smith (2000), and Joel Shapiro (2001). Yesterday it was given over to Conceptual artist Sol LeWitt, whose outdoor sculptures can also be seen beginning May 1 at Madison Square Park.

Some works fare better in nature than others. I visited the Cantor Roof Garden on Monday, under a big, blue, and windy, nearly cloud-filled sky; the park’s treetops rolled out from the deck’s railings in a budding, hazy sea of pinks and yellow-greens. In this kind of environment, the art of Mr. Kelly (which can feel like a distillation of leaf, bud, and bird’s swoop) or Smith (whose iron contraptions remind us as much of animal and man as of machine) spoke to the modern era and to Mother Nature in equal measure. Their artworks were in conversation with the city and the park, as well as with the art below deck in the museum.

By contrast, “Sol LeWitt on the Roof: Splotches, Whirls, and Twirls” – the slick grouping of five painted Fiberglas sculptures and one mural painting, “Wall Drawing #1152 Whirls and Twirls” (2005), all of which were executed by assistants – are impersonal pastiches. They are as removed from the beauty and the operations of nature as they are from aesthetics.

The syncopated wall painting is a decorative eyesore, a throwaway work made of meandering, hard-edged, brightly colored shapes in red, yellow, blue, yellow-green, orange, and purple. Each sculpture is somewhat different, but basically they all look the same. They look like the elastic shapes chewing gum assumes when – to exclamations of “eeeeeew!” – a sticky wad is lifted over and over again from the hot summer pavement by the sole of your shoe.

The 6-foot, 6-inch tall “Splotch #8” (2002) is all black and, because it reflects the blues of the sky, at least has some surface interest. It looks disturbingly like a melted Darth Vader. “Splotch #7” (2002) is the same size but all white and reminiscent of an iceberg. “Splotch #3” (2000), at 12 feet long and nearly 4 feet tall, could be said to relate to the uneven rhythms of the skyline. But this is only because the uneven rhythms of the skyline relieve you from the aimless anxiety of the sculpture’s EKG-like peaked contours.

The problem is not that Mr. LeWitt’s artworks at the Met are dissatisfying, formless, and soulless. They are Conceptual art, and Conceptual art, by its very nature – if it can be said to have one – is not supposed to be attractive, satisfying, or alive. We should not be asking: “Why, exactly, did the Met give over its roof garden to Mr. LeWitt’s mindless, candy-colored blobs?” The larger question is: “Why, finally, 40 years after the fact, can’t Conceptual art stay at the level of ‘concept,’ where it belongs; Why do we actually have to keep on seeing it?”

During the 1960s, Sol LeWitt (b. 1928) made the miraculous discovery that all art begins with an idea – a “concept.” “In Conceptual Art,” he wrote in 1967, “the idea or concept is the most important aspect of the work … all planning and decision[s] are made beforehand and the execution is a perfunctory affair.” This started a “Conceptual” revolution in art: Artists started making art that, ironically, could be made by other people. This freed them from the responsibility for the artworks’ ultimate degree of success or failure as actual functional objects in the real world. Sometimes, it was decided, the art was better left not made at all.

Yet they kept on making it. Mr. LeWitt has made a whole career out of coming up with art “ideas” – murals, drawings, empty cubes, books, sculptures – that are produced by other people. But, oddly, Conceptual artists choose not to eat Conceptual food, see Conceptual doctors, or make Conceptual love – activities that do not appear to fall into the “perfunctory” category. It seems that the Conceptual begins and ends with art.

It doesn’t, and Mr. LeWitt appears to be torn between ideas that produce Conceptual art and ideas that produce things that resemble actual art. He wants to eat his cake and have it be Conceptual, too. This is certainly the case at Madison Square Park. His two sandblasted, light-gray cinder-block sculptures are too bland, impersonal, and unfinished – too hard, stark, cool, and bare – to be ultimately engaging. But they actually relate in small, pure ways to the surrounding trees, park, and architecture.

I am not advocating that the two sculptures should be permanently installed. I am not even advocating that they will hold anyone’s interest long enough to warrant the cost of materials. But there is something more to them than there is to the works at the Met.

One sculpture, the 14-foot-high, 25-foot-in-diameter “Circle With Towers,” is a cement-block ring, out of which eight towers rise at equal intervals. It is at the southern end of the park. The second, an 85-foot-long S-shaped sculpture with 17 towers, titled “Curved Wall With Towers,” looks like “Circle With Towers” stretching out and going for a stroll. It is at the eastern side of the park. As I walked around the sculptures, they enticed me to get closer, like ruins. Both works feel like extensions of the park’s curving geometry: Their towers, fragmentary in feel, resemble supports for unknown rituals or for larger works of past or future architecture.

As art these sculptures fall far, far short of the mark. But in their purity – their exposed, raked joints and measured, square corners; their evenly pockmarked surfaces; and their cross between furniture, doorway, and prison wall – they suggest a non-Conceptualist Conceptualist may be behind them after all.

“Sol LeWitt on the Roof” until October 30 (Metropolitan Museum of Art, 1000 Fifth Avenue, at 82nd Street, 212-535-7710).

“Mad Sq. Art: Sol LeWitt” from May 1 until December 31 (Madison Avenue, between 23rd and 26th Streets, 212-538-4689).