'Something there is that doesn't love a wall," wrote Robert Frost. Not in Philadelphia, my good man.

This city loves blank walls, for if you have a wall with an open space in front of it, you can cover it with a painted mural that will delight some people and annoy others.

Since 1984, when it began as a component of the Philadelphia Anti-Graffiti Network, the Mural Arts Program has created about 3,600 murals all around the town, which has given rise to the sobriquet "City of Murals."

You've no doubt encountered some of them, or at least heard or read the encomiums the program has received over its three decades. This attention is well-deserved, not so much for the aesthetic quality of the product, which varies, as for the salutary impact that this socially conscious mural-making has had on neighborhoods and the lives of more than 30,000 young people who have participated.

For the program's 30th anniversary year, the Pennsylvania Academy of the Fine Arts has organized a show called "Beyond the Paint." As PAFA museum director Harry Philbrick acknowledges, it's not a traditional art exhibition: "It is a didactic, documentary experience."

Visitors will notice this immediately, and also that, except for some work by schoolchildren, it doesn't contain any original art. Instead, one is treated to a steady progression of photo-storyboards. If you use reading glasses, remember to bring them.

What one takes away from "Beyond the Paint" depends on whether the subject is approached as art or social work. The murals chosen for this book-on-the-walls are, for the most part, professionally executed, and sometimes striking.

In spirit, whether hortatory or commemorative, they tend to be propagandistic and boosterish; collectively, they extol mural-making as a civic virtue. The fundamental issue, which an exhibition of this type can't address effectively, is: How do they play in wide-open spaces?

Compositionally, the murals that have the most impact, regardless of scale, are those that project a single strong image against a neutral ground.

The full-length portrait of a debonair Julius Erving at 12th and Ridge, by Los Angeles artist Kent Twitchell, and local painter Sidney Goodman's figure of a young African American boy with one arm raised near 40th and Powelton are superb examples of this type.

More complex compositions - the majority of murals portrayed at PAFA - are not only harder to read at a glance, they tend to camouflage the buildings on which they're painted, in the way that advertising "wraps" make buses invisible.

A striking example of this phenomenon is the quotation of Katsushika Hokusai's "Great Wave" on the side of Bodine High School in Northern Liberties. The architecture and the painting struggle for supremacy, resulting, ironically, in the kind of visual confusion that graffiti creates.

As its contribution to the program's social-work dimension, the show includes a mural-making studio where visitors can meet muralists at work on projects and even participate in the process. Yet, at the end of the tour, one feels that a filmed documentary that included citizen interviews, or even a real book, would have told this remarkable story more directly and effectively.

Seeing isn't believing. Speaking of his own painting, Frank Stella said, "What you see is what you see." At the Fabric Workshop and Museum, Sarah Sze puts a twist on that famous aphorism - something like, what you see can be both more and less than reality.

The 44-year-old artist, who represented the United States at last year's Venice Biennale, is a conceptual manipulator of space, mass, volume, and scale, all important components of visual perception.

At the workshop, she has created three sculptural installations that not only challenge how humans interpret visual information, but also suggest how perceptions can be shifted to make familiar objects become fancifully poetic.

Perhaps the easiest of these to grasp is the "boulder field" that fills the second floor. Some of these rocks are so large that one immediately wonders how Sze (pronounced ZEE) got them into the building.

The answer: The boulders are faux - constructed of Tyvek fabric printed with photo-derived textures of real stones that's been stretched over aluminum or polystyrene armatures. Their scale is perfect, their volumes convincing, and their surfaces so true that they fool the eye even from a few inches away. But mass, which the eye can't measure, is negligible.

A piece on the eighth floor proceeds from reproductions of New York Times front pages spread across the floor. Sze has replaced all the news photographs with images of the sky or what appear to be lunar or Martian landscapes. This simple substitution transforms the quotidian into the cosmic - half familiar, half mysterious.

Sze's most provocative and stimulating piece is the "exploded desk" on the first floor. Using the workshop's reception desk as a model, she produced a multidimensional, cubistic version out of thin metal rods.

Utilitarian furniture morphed into delicately abstracted architecture best communicates the ingenuity and delight of Sze's imagination.

Of Walls and Boulders

"Beyond the Paint" continues at the Pennsylvania Academy of the Fine Arts, 118 N. Broad St., through April 6, 10 a.m. to 5 p.m. Tuesdays through Saturdays and 11 to 5 Sundays. Admission: $15 general; $12, 60 and older and students with I.D.; $8, 13 through 18. Free Sundays during the run of the exhibition. 215-972-7600, www.pafa.org.

"Sarah Sze" continues at the Fabric Workshop and Museum, 1214 Arch St., through April 6, 10 a.m. to 6 p.m. Mondays through Fridays, noon to 5 p.m. Saturdays and Sundays. Admission: $3; children under 12, free. 215-561-8888, www.fabricworkshopandmuseum.org.