At the moment, the world is entranced by a new kind of ''cubism'' -- the Rubik kind. That little plastic toy, with its multi-colored cubes that twist and turn into multi-billions of combinations, actually gives us a clue to the first kind of ''Cubism'' -- the Picasso kind, circa 1907.
It all started with Cezanne who saw the world in terms of geometry. Nature, the human body, and everyday objects could all be rendered into the simple components of sphere, cylinder, cone and, yes, cube.
To be truthful, the term ''Cubism'' was coined by an art critic who needed a word in a hurry to describe what he was seeing in the paintings of Picasso and George Braque in the early part of this century. Cubism is not actually so square as to limit itself only to the cube and in fact that basic component of most cubist paintings is the ''plane,'' the most elementary geometric unit next to the point and the line.
As the Rubik cube consists of small cubes making up the form of a large cube, so Picasso's Cubism translates figures, still lifes and landscapes into the common denominator of interlocking planes.
The drawing pictured here, ''Seated Musician'' (1912), is a rudimentary Cubist sketch by Picasso, though a fairly late one in the Cubist movement which ended as World War I began. Compared to such paintings as ''Man with a Mandolin'' (1911) in which the full force of the Cubist idea exploded onto the canvas in mind-boggling complexity, this sketch is a simple exercise.
For one thing, background is totally ignored. The interaction between an object and its background was one of the most developed themes in Cubism, but hardly the only one.
What we certainly have here is Picasso's Cubist vision of the human figure, the ultimate man -- to the third power. Taken as a metaphor, it melds man and mechanization. Taken as art, it shows yet another way to sketch the human figure.
Art like this certainly shows us what the ''eye hath not seen'' in the natural world. The Cubists firmly believed that painting and drawing images which had never been seen before would expand and renew people's concepts of themselves and the universe.
Many people are disturbed by such distortions of the human body as Picasso regularly accomplished on his canvases and sketch pads. The head of this figure is perched like a boulder on a ledge; it is one of the few curved things about him. A whimsical staircase of planes forms his chest.
But the angularity of Cubism soon enough dissolved into the liquid forms of Picasso's Surrealist period. Picasso's distortions are usually purposeful. They are experiments in perspective, explorations of the possibilities of form, insights into the extremes of human emotion.
Picasso said, in explanation: ''I paint forms as I think them, not as I see them.'' Cubism was a mental investigation into the structure and relationship of forms. It was made visible in what we call art. Though some of the more advanced Cubist paintings seem to have become cliches of bewilderment, the logic of Cubism remains clear. Of Cubist figure painting, the art critic John Berger said: ''. . . the structural arrangement which the body inhabits is made as tangible and precise as the architecture of a town.''
Cubism was and is an art of optimism. It was born at the same time and coincidentally incorporated many of the same assumptions as modern physics. As scientists took apart the atom, so Picasso, Braque and others who followed them took apart the appearance of forms into their component planes.
Picasso's biographer, Roland Penrose, says of the Cubist vision: ''Geometric simplifications adopted instinctively by the artist allowed him to create new harmonies with the same precision whereby crystals form with the frost.''
Whether an art of ''sphereism'' is possible is difficult to say. What Cubism showed the world was that art is not just concerned with the appearance of things, but with new images that come from a world of endless possibilities.