Asked for a definition of abstraction, the late art critic Harold Rosenberg once replied, ''Abstraction in art is the same as in life. It's a process of refinement, of eliminating what you can do without.''
As applied to ''Design Since 1945,'' an exhibition currently on view at the Philadelphia Museum of Art, his definition assumes visionary force.
The core of this exhibition, a major international survey of postwar consumer design, is functionalism. And it is a most exhilarating exhibition. Even if you don't decide to throw out the contents of your house and start over, its effect is to educate the eye to the home environment and open the mind to a purer form of design.
Functionalism was the dominant design aesthetic of the modern period, originating with the Bauhaus School in Germany and advocating the integration of form and function.
Kathryn B. Hiesinger, a curator of European decorative arts after 1700 and organizer of the exhibition, explains functionalism in her catalog essay as ''the theory that beauty in useful objects is defined by their utility and honesty to materials and structure.'' In other words, ''less is more.''
Functionalism cannot be separated from the abstract movement in painting and sculpture, or from the International Style in architecture. It gave rise, as this exhibition so dramatically illustrates, to design that disdained all decoration as distraction from the essential form of the object. The most pernicious threat to this standard was gratuitous change for the sake of change or planned obsolescence.
Mrs. Hiesinger summarizes the history of home design throughout the past 40 years as follows: the postwar years, when furniture was of necessity ''light, portable and economical''; the '50s, when Scandinavian design, with its emphasis on wood and natural materials, coincided with a more informal life style; the ' 60s ''explosion'' from Italy, with its emphasis on bright colors and practicality; then the '70s, in which the Italian Memphis group led by Ettore Sottsass Jr. proselytized not only a revival of decoration but a populist aesthetic that was responsive to changing societal needs. All of which leads up to the contemporary vacuum, according to Mrs. Hiesinger, which is waiting to be filled.
It's a dynamic and complex history, and this exhibition captures the adventurous and experimental flavor of design during this period.
On display are some 450 examples of furniture, lighting, glass, ceramics, metalwork, textiles, plastics, and appliances representing 208 designers from Italy, Scandinavia, Japan, Britain, the United States, and other countries. Every object bears a relationship to functionalism, whether as example or reaction.
George Nelson, an internationally known designer (of the pedestrian mall and the room-divider storage wall), has arranged the exhibition to emphasize connections rather than chronology. Discrete sections are devoted to lighting; furniture; ergonomics (the scientific study of how best to adapt tools and objects to suit an individual or worker); technology and industrial design; environmental pieces, such as Joe Colombo's portable kitchen; and the history of modern tableware.
The exhibition is a diverting conglomeration of the familiar and the unfamiliar, the banal and the esoteric, the handcrafted and the mass-produced, the durable and the disposable.
The first section is a curved corridor of lamps that flicker on and off in alternate groups. The most famous lamp is virtually an international fixture, Achille and Piergiacomo Castiglione's ''Arco'' floor lamp, an arced stainless steel stem rooted in a marble base from which the globe hangs like a pendulous lily. In contrast with ''Arco'' are the geometric designs borrowed from machinery and technology, such as Gerald Abramovitz's ''Cantilever Light'' or Gino Sarfatti's ''Table Lamp,'' an homage of sorts to the vacuum cleaner hose.
Similarly, in the furniture section many of the most celebrated chairs - by Charles Eames, Arne Jacobsen, and Eero Saarinen - seem molded to the contours of the body, while others are more bone than flesh in inspiration - skeletal shapes in wood or metal that evoke the bareness of a wire sculpture.
With the exception of the ergonomic chairs, this furniture, however witty, refined, or sculptural, is definitely not for comfort or relaxation.
Surfaces are generally cold and hard, textures artificial (vinyl, plastic, fiber glass, even cardboard). The overall impression is one of rigidity even when the lines are flowing. One finds oneself longing for grandma's ugly, overstuffed chintz armchair into which one sank six inches before settling just above the springs.
It's no wonder that tapestry became so popular as a complement to this furniture (many examples of which are on display, notably Jack Lenor Larsen's), as the senses demand some softness.
The ''high-tech'' section of the exhibition, a circle of monumental pylons deliberately reminiscent of Stonehenge (according to Mr. Nelson), features the dizzying progress of technology through the progress from the adding machine to the ''compass'' computer; the electric mixer to the Cuisinart; the 1959 Philco, with its Cyclopean eye, to Marco Zanuso's ''Black 12'' television, a pristine black cube that masks its true identity.
These juxtapositions make clear that the direction of recent technological design is toward spareness, economy, and efficiency, a tendency that, along with the influences of modern art, has shaped the sculptural simplicity of consumer design.
The display of tableware is a diverting and at times amusing conglomeration of the recent history of the international table. Some of the objects, such as Tupperware, are quite commonplace.
There are also more exotic (and expensive) pieces, such as Henning Koppel's sterling silver dish, so sculpturally fluid that it seems more fish than dish.
The exhibition, funded by Best Products Company Inc., the Pew Memorial Trust, and the National Endowment for the Humanities, will run through Jan. 8. Videotape programs, a symposium, and lecture series supplement the exhibition.