The Moholy-Nagy paintings were pretty cool, but nothing beats the hanging and standing plexiglass sculptures at the recently closed Moholy-Nagy show at the Guggenheim. You get something of the idea in his Vision in Motion (1947). Moholy-Nagy (pronounced Mohly-Naagh) describes five stages in sculpture:  the barely modeled, solid block, almost untouched,  the modeled block,  the perforated block,  the completely perforated block, and  the mobile (pp.220-38) . You can find here the whole book, which bibliophiles, antiquarians, and design artists will find typographically spectacular.
The idea of a completely perforated block is utopian in character, a theoretical construct which contemporary sculptors were still searching for adequate means with which to build. It is described by the artist as a “bold sublimation of the material.” Free from relationships to external points, it was meant to float in space, conquering gravity. Ideally, it was meant to be held up not by the illusion of wires, but by “magnetic forces or remote electrical control” (p.236). As for the mobile, it is supposed to convey the same effects, namely the reduction and lightening of a heavy mass in which the normal characteristics of the material disappear. The material element’s function is now a “carrier of movement” or a “carrier of forces” (pp.237, 238).
As the conversation develops in Vision in Motion, Moholy-Nag considers the “duality of volume.” In fact, he names three aspects, not two. These are volume as a “clearly circumscribed mass,” negative volumes consisting of void, holes, and openings perceived through the limiting walls of the circumscribed mass, and “virtual” volumes produced by the motion of points (the smallest possible bodies) or by the motions of linear elements. It is the virtual volume that is the new element in the forms of plastic creation made possible by new technologies (p.238).
For all that Moholy-Nag rejected metaphysics and “the spiritual in art,” his emphasis on sublimation and dematerialization remain caught within the mesh theorized by Kandinsky. These sculptures convey the sense of new spiritual entities. Moholy-Nagy called these plexiglass works “space modulators.” They are meant to “activate space” (p.238) by casting light and shadows. Against type, the more earth bound figures have a tragic grandeur. Key to these works is the cheap plexiglass, which the artists was able to heat and contort.
Alas, these sculptures do not get their full due as ethereal objects in the otherwise excellent catalog, Moholy-Nagy: Future Present. The space modulators are not to be confused with the weapon wielded by Bugs Bunny nemesis Marvin the Martian, the Illudium PU-36 Explosive Space Modulator.