Reading BJ Leggett's excellent (& nice & short) Late Stevens. I really liked one of his earlier books, Wallace Stevens and Poetic Theory. He has a way of correcting (gently) the Stevens big shots, Bloom & Vendler et al.
Stevens, in a statement on "Auroras of Autumn" : "There are many poems relating to the interactions between reality and imagination, which are to be regarded as marginal to the central theme... [which is] the possibility of a supreme fiction, recognized as a fiction, in which men could propose to themselves a fulfillment." (quoted by Leggett on p.1). & from the epigraph to 1st chapter : "The exquisite truth is to know that it is a fiction and that you believe in it willingly." (from Stevens' Adagia)
Leggett's argument (which I'm just getting going with) is that many of Stevens' later poems (in The Rock, etc.) are intertextually aligned with "Notes Toward a Supreme Fiction" and other, earlier poems on this "central" theme. & (to paraphrase) Leggett summarizes Stevens' focus on this theme, as a meditation on the idea that the fiction of reality is the generating dream of an inhuman mind - sagacious, inventive, changing, unaccountable, inexplicable... yet - as an imagination - showing a likeness to our own imagination. Leggett attends closely to some lines in the opening poem in The Rock, "An Old Man Asleep" :
The two worlds are asleep, are sleeping, now.
A dumb sense possesses them in a kind of solemnity.
The self and the earth - your thoughts, your feelings,
Your beliefs and disbeliefs, your whole peculiar plot.
The redness of your reddish chestnut trees,
The river motion, the drowsy motion of the river R.
He shows how these lines imply that the river & the chestnut trees are the thoughts of one of the "two worlds" (the earth) - that is, that the things of the earth, the "outside", are fictions of a greater, "inhuman" dreamer; & the "two worlds" are mingled together here, ambiguously joined.
So it seems the "supreme fiction" is a new expression of a very old idea... of the fictiveness (the "made" quality) of reality.