But what does it mean?

Imagination is a splendid thing. Art, in many ways, couldn't do without it. We tend, however, to misunderstand it, and to see it primarily in unreal, fanciful, or exotic terms. Thus, dragons, elves, and unicorns are said to be products of our imagination, and a child who talks to an invisible playmate is described as having a vivid imagination.

Imagination is seen as a fanciful alternative to, or as an escape from reality. Neither it nor its products, thus, are to be taken altogether seriously. Real life, as we all know, is strictly a matter of common sense and logic.

But imagination is much broader and more important than that, and ranks among the most valuable of all human attributes. It took imagination, after all, to come up with the idea for the first wagon, or to perceive the importance of farming, cooking, or building a home. Imagination is what Durer needed to conceive a more objectively drawn rabbit than any drawn before, and what Stieglitz required to take photographs that were works of art.

Art and imagination have always had a close and dynamic relationship. Art, after all, engages us imaginatively. It provokes us into perceiving alternatives to what we know and feel, and into accepting at least something of what it conveys. Its function is

dynamic, not static. It doesn't exist merely to look pretty or impressive but to utilize those qualities and any others it can to challenge and to engage our imaginations, and thus to expand and enrich our present and future lives.

Imagination at the service of art can accomplish extraordinary things, but only if the artist permits the viewer to exercise his or her imagination as well. Little is accomplished if an artist uses great imagination to fashion something that cannot in turn stir the imagination of its viewer.

That stimulus can vary greatly. In the hands of a Michelangelo, it stirs us toward feelings of grandeur. In Rembrandt, toward feelings of depth and compassion. And in Cezanne, toward a remarkably holistic sense of order.

But there are also subtler and more deeply interior provocations, such as those found in the works of Blake, Redon, Klee, and Rothko. In addition, Morris Graves's birds and animals resonate with and direct us toward philosophical ideas, and the best of the Conceptualists cause us to question the very nature of perception and experience.

There are artists everywhere whose one big dream is to distill something universal from the dense and complex particulars of their lives. This dream may vary in detail, and in the style and form such distillations would assume as art , but it remains constant on at least one point: its wish to extract a significant response from the viewer.

But how does one accomplish this, especially if the artist's ''distillation'' is paticularly subtle or open to more than one interpretation? If the resulting image is too precise, the viewer may miss the subtler implications of what the work is intended to convey. But if it is too vague or obscure, the viewer may miss its point entirely.

Philip McCracken, a sculptor of great warmth and imagination, has found a way out of that dilemma. And he has done so even though some of his ideas are extremely subtle, and a few cannot adequately be translated into words.

The first thing that strikes one about McCracken's work is that every piece of sculpture represents an extraordinary fusion of form and idea. This fusion is so perfect, so inevitable, in fact, that it almost seems like the rejoining of two halves of a whole that had somehow broken apart.

Form and idea are so evenly matched and so irrevocably joined in his art that it is impossible ever to separate them. They exist as one, and they achieve a sort of perfection thereby.

That, however, is the only kind of perfection one will find in McCracken's work. He simply is not interested in perfection in absolute or transcendent terms - at least not as far as his art is concerned. His interests are more basic and elemental, more focused upon resolutions and reconciliations, and upon the maintenance of a civilized balance between extremes.

This desire for balance was already very evident in his early sculptures of animals and birds, executed in the 1950s. There is a simplicity to these embodiments of whales, owls, frogs, rabbits, and other creatures that is quite extraordinary, and that relates more to the art of Brancusi than to that of Henry Moore, whose assistant McCracken had been for a while.

Balance became an even more critical ingredient of his art during the late 1960s, when he began probing into increasingly complex and ambiguous areas and dimensions of meaning.

For the rest of that decade, McCracken's art remained primarily Conceptual in orientation, and somewhat menacing in implication. But here again, his objective was to make a point, not to indulge in histrionics. If his images were aggressive and pointed to painful truths, or if they raised questions to which there were no easy answers, it was only to challenge his viewers to exercise their imaginations toward very specific ends.

One of the best of these works is entitled ''Lights Out.'' It consists of five little bulbs centrally positioned in a transparent box. Four of the bulbs have evidently been smashed by bullets whose holes are dramatically visible. Only one bulb remains intact.

It's a provocative image - but what does it mean? The title tells us very little, and absolutely nothing about who did the shooting and why.

If we want a clue to this work's meaning, we'll have to find it by ourselves. And to do that, we'll have to exercise our imaginations.