In my book, The Illusion of Will, Self, and Time, I cite Nietzsche's calling out "will" "self" and "causality" as "phantoms" of the "inner world." And phantoms they would all indeed need to be if Eternal Recurrence is real. Scholars keep trying to limit Nietzsche's engagement with Eternal Recurrence to a thought experiment. But it was clearly far more than that for him, as I set forth in my book:
--begin quotation from The Illusion of Will, Self, and Time
Nietzsche’s passionate promotion of eternal recurrence as a reality, rather than merely a doctrine he had been familiar with and written about as a Greek scholar, came to him suddenly during a walk in the woods, as he approached “a powerful pyramidal rock” (Nietzsche, 1908, 295). The original mystical insight of this ecstatic vision supersedes the implausible physics he later tried to supplement it with: finite matter rearranging itself in infinite time into perpetually recurring exact replicas (see Nehamas, 1985, 144–145); see also, Kaufmann: Nietzsche’s “reasons for not publishing a proof [based on physics] presumably included his own sense that his efforts were inadequate” (1950, 327). For such added-on physics were not “presuppositions that would have to be true if it were true” (Nietzsche, 1901, 545; emphasis added). The only presupposition of physics “that would have to be true” is what Bohm identified as physics’ real fact: an “order of succession.” (Bohm, 1992, 233). The physics that does support eternal recurrence, special relativity, emerged five years after Nietzsche’s death [see Frassen (1962) contra Capek (1960)]. Attempts to construe Nietzsche’s “highest formula of affirmation” as a “thought-experiment” (Arendt), a “pretend” game (Sartre) (Lukacher, 1998, for both, 117), or, most recently, a “grand fiction” (Panaïoti, 2013, 128) run counter to “the strong emotion of the discovery” that left him “bathed in tears” for a long time (Halevy, 1911, 231). His beloved companion Lou Andreas-Salomé’s account of how he experienced it confirms as much: “To me the hours are unforgettable in which he first confided it to me, as a secret, as something he unspeakably dreaded to see verified . . . : only with a soft voice and with all signs of the deepest horror did he speak of it. And in fact he suffered so deeply from life that the certainty of the eternal recurrence of life had to entail something ghastly for him” (Lou Andreas-Salomé in Löwith, 1997, 197–198). These are not the expressions of experiment, pretend, or fiction.
Bohm, David. (1992/1994). Thought as System. New York: Routledge.
Capek, Milic. (1960). “The Theory of Eternal Recurrence in Modern Philosophy of Science, with Special Reference to C.S. Peirce,” The Journal of Philosophy, 57 (9), 289–296.
Halevy, Daniel. (1911). The Life of Nietzsche. Translated by J. M. Hone. London: T. Fisher Unwin.
Löwith Karl. (1997). Nietzsche’s Philosophy of the Eternal Recurrence of the Same. Translated by J. Harvey Lomax. Berkeley: University of California Press.
Lukacher, Ned. (1998). Time Fetishes: The Secret History of Eternal Recurrence. Durham: Duke University Press.
Nietzsche, Friedrich. (1901/1967). The Will to Power. Translated by Walter Kaufmann. New York: Random House.