The irrational bears the same relation to the rational that the unknown bears to the known. In an age as harsh as it is intelligent, phrases about the unknown are quickly dismissed. I do not for a moment mean to indulge in mystical rhetoric, since, for my part, I have no patience with that sort of thing. That the unknown as the source of knowledge, as the object of thought, is part of the dynamics of the known does not permit of denial. . . . We accept the unknown even when we are most skeptical. We may resent the consideration of it by any except the most lucid mind; but when so considered, it has seductions more powerful and more profound than those of the known.

—Wallace Stevens, circa 1937

What was the nature of the quest that moved the Dutch painter Piet Mondrian (1872–1944) to abandon the representation of nature in favor of an art of pure abstraction? What, exactly, did Mondrian believe that he had achieved? In any attempt to address this question, we are obliged to deal with the fact that abstract art—and not only Mondrian’s—was born of an alliance of aesthetics and mysticism. We are obliged to examine the ideas that shaped the artist’s search for the absolute in art. Ideas, of course, are no substitute for the experience of art, and in Mondrian’s case are certainly not to be taken to be the “subject matter” of his painting. Yet without some