Being placed on top of a hill gives the jar an apex of human purpose through nature. But the jar asserts authority even more through the implied design of its own rotundity. It is the design of a created object embodying a human, cultural purpose. Further, the roundness is the symbolic design of purpose placed in nature, which in itself lacks purpose or order. The jar's roundness, exerting a centripetal force on the "slovenly wilderness," endows the wilderness (including the hill) with the order of a center. All the natural disorderliness of the wilderness acquires a purposive spatial character through "centering," and is given a figurative order in the way "rounded" and rounding human purpose shapes significance into the raw matter of earthly phenomena. Accordingly, human circularity, human centralization, civilizes "wilderness," not only the wild, that is, but chaos, nullity, meaninglessness, by providing it structure. This governing force is so powerful that even in its plainest, simplest representations ("grey and bare") the jar compels a "surrounding."
"Anecdote of a Jar" is a metaphor about the magnetic power of mind and art to order a void (and the void). Stress is laid upon its non-naturalness (11.10-12) to accentuate the crucial power of artistic and thus human purpose. Art (mind) governs its antithesis, nature—"It took dominion everywhere," even, indeed, especially, in a non-civilized, non-human place.
The shaper here is himself "round"; he rounds significances through symbolic artifacts which express his desire to dominate all that is senseless or shapeless or wild by compelling it into pattern, thus transferring from God to art and man the force of the ancient gnosis that "God is a Circle, whose Circumference is nowhere and whose Centre is everywhere."
From "Circular Art: Round Poems of Wallace Stevens and William Carlos Williams." Concerning Poetry 14:1 (Spring 1981).