I've rarely read a discussion of a poem I disagree so strenuously with as Nada Gordon's discussion of this. She reproduces the text in full on her blog, freeing me from the temptation to do so here, and then she writes
It’s a confusing poem, probably deliberately so, and I felt after reading it and discussing it that it doesn’t transcend its own contradiction: the cry “seem[s] like a sound in [his] mind” but he maintains, in that weird conditional tense (and with a potentially ambiguous pronoun reference) “It would have been outside”.…The poem seems perfectly clear to me; it's about the coming of death, the ultimate "thing itself"; death whether literally or as the embodiment of any realization of the complete whatness of something. The poem is no more "confusing" than the difference between seems and is; and "would have been" is not "conditional tense" [sic] but laconic spoken English. Wallace Stevens is so readily thought a mandarin that his most direct discourse is analyzed into opacity; it's an example of the danger of reading intellectual complexity into material that is, in fact, perfectly straightforward.