Deleuze always comes back to the eternal return as the high point of his attempt to think of difference in-itself as repetition for-itself. As befits the content of the concept, each time is appears it has a slightly different form. This time, the emphasis is on it's relationship to possibility and to a pure chance the removes all traces of the concept of necessity. Eternal return is linked to the death instinct, but the death it implies liberates a sense of play and invention. As Nozick put it in Invariances: "only lack of invention is the mother of necessity".
At first sight it's not obvious that the ER would be opposed to the idea of necessity. On the contrary, if we understand it as the proposition that at some point everything will repeat again in identical fashion, it seems to suggest that absolutely everything necessarily must be just as it is and was, as if it were all part of God's inscrutable plan for the universe. [Note: I am going to use "world" and universe" synonymously here] After all, this is not even the first time it's all happening, right? However, no philosopher has ever worked as hard to be misunderstood as Nietzsche, and ER has to be his most misunderstood concept. In fact, the idea is meant to lead us in exactly the opposite direction of a fixed and necessary repetition by forcing us to ask some questions about how that would work. It takes quite a bit of work to see this reversed perspective though.
You ask yourself, for example: when did the latest cycle of the universe start? Obviously, the answer is: after the last one ended. So, maybe when time ends, it repeats again from the beginning. The form of time and its contents would both be both finite and repeating. Or maybe, rather than starting again, we might instead conceive of time as progressing in an infinite straight line. Since matter and space are finite though, if things keep moving around long enough they will inevitably end up back in the starting position (at least that's the intuition, I rather liked George Simmel's counter-example here). In that case the form of time would be infinite, but its contents are finite and repeating (which when you think about it kinda defeats the idea of its form being infinite, see below). These are two not actually very different flavors of how the universe could repeat itself.
Given our ideas about the big bang and the heat death of the universe, they may sound plausible to the modern ear. We might wonder whether it repeats again exactly, with the exact same initial conditions at the Big Bang leading, via the inexorable laws of physics, to the exact same evolution of the universe, right down to me typing this exact same sentence with it's reference to G-Eazy, followed immediately by zombie apocalypse and heat death. But, regardless of whether we start with the same initial conditions every time or whether we try to argue that ER is wrong and no two universes are alike because of tiny random variation in the initial conditions, the basic idea seems to make conceptual sense -- 'everything' could repeat.
It make sense to us because we are so accustomed to the basic assumptions of a materialist worldview. We situate ourselves outside of the world, looking at it from above as a closed totality with beginning and end, governed by specifiable laws that are also outside that world. What's repeated empirically (or not) would be the universe as a whole. And what's repeated transcendentally, as a condition necessary for thinking about the first repetition, are the identity of the immaterial laws that govern it. It's only from this viewpoint outside the material world that we can stand back and see what counts as a full cycle and count the number of times it has repeated. This is the perspective that gives time a hinge that divides one repetition from the next. This is the only perspective from which we can imagine running the experiment of the universe a second time.
That all seems like a perfectly coherent perspective till I write it down. I mean that literally. Because as I write it down, I somehow appropriate that external perspective for myself. Yet, the last time I checked, I was still just a hairless chimp floating through space on an unremarkable rock, seemingly as much a part of the universe as anything else. So how did I, or any part within this universe, manage to attain the perspective of looking at it from without, the only perspective that would enable me to see whether or not it repeats?
Or maybe more succinctly, we might ask: how did I write this whole blog post down the first time? After all, the first spin of the merry-go-round was not a repetition. The unit of repetition had yet to be formed. So of course that time I was not busy writing about how it repeats, because it hadn't yet. But, well, I'm writing this and you're reading it, so either this is indeed the first time, or this time is somehow different from it precisely because I'm here identifying the repetition. In either case, my thought that the universe repeats is true it would count as evidence that it does not repeat.
Or maybe we think there was no first time, and the unit of repetition has simply always been there. I've always sat here writing this exact same line, and you've always sat here reading it. But then, how would we know? How would we ever figure out which number repetition we're in? We would have to have some memory of the last time around with which to confirm the identity, but like we saw with Hume, this would add a privileged point or extra dimension to the experience that makes each time around different. But if this unit is simply all there ever is over and over again, and there is no overlap or static supplementary dimension, doesn't the whole idea of a repetition lose all meaning?
It kinda reminds me of the sophomoric notion of base reality. Sure, maybe we're in a simulation instead of base reality. But then again maybe it's just a simulation of a simulation. Or etc ... And how would you know the difference between base reality and a simulation, or one simulation and another, unless you find some way of passing from one to the next? And even if Elon Musk perfects astral travel before he gulps another red pill, how would one level announce itself as more real or base than any other? The act of considering possible simulations renders the concept of base reality meaningless.
The point with all this analytic rambling is that if there's someone who can think this thought or write this very sentence, then the idea that the universe repeats exactly becomes internally incoherent. There's a non-obvious paradox built into the eternal return, a self-referential structure perhaps a bit like the diagonal method or Gödel's proof. This is why it comes up again here in the context of the death instinct, which for Deleuze is associated with a dissolving self-referential process.
As Klossowski says, it is the secret coherence which establishes itself only by excluding my own coherence, my own identity, the identity of the self, the world and God.
But just when you thought this had gotten dizzying enough, you can see in this quote the glimmer of yet another twist. The expression of the idea that the history of the world is a unit that necessarily repeats seems to dissolve that very idea. The natural contrary would seem to be that the world must only unfold one time, once and for all, as a completely unique entity. Except, part of that universe is busy imagining that the universe, and it itself, repeats. You might say that this is just some sort of hallucination. This is the simplest refutation of what I wrote above -- the idea that the universe repeats just isn't true. But now your base reality must contain perfectly real hallucinations, objective illusions. These hallucinations actually work just like little simulated thought images of our whole universe, except with the added difference that in those simulations it is all happening again the same way. But of course that makes it different from the perspective of the thinker of the simulation, who has to hold reality and the simulation together to image their relationship as one of repetition. So if we have just one universe in which the question of the eternal return has been asked, it suddenly fragments into miniature copies of itself, each of which is actually a different repetition of the whole, within that whole, ad infinitum. Even if that question is an illusion or a hallucination, it still produces this fragmentation as real effect, and in fact, our only approach to this whole is through these hallucinations that are now objectively part of it.
The world is neither finite nor infinite as representation would have it: it is completed and unlimited. Eternal return is the unlimited of the finished itself, the univocal being which is said of difference.
It's difficult to talk about the eternal return because it has this strange circulation built directly into the idea. Paradox in general has this sort of effect. "This statement is false". If it is, it isn't, and if it isn't, it is. There's an internal movement or restlessness. With a simple paradox that doesn't get you anywhere because you just move in a circle. But with ER, you get back a new world every time, but in the form of the possibilities that were already inherent in this one. If I think that the world repeats then it must not, but if it doesn't then my thought that it does creates a repetition that didn't exist before, and then this cycle repeats, splitting into new worlds again and again. The thought of a unique world repeating is the thought of an infinite multiplicity of different worlds.
When we asked who annihilates themself with the death instinct, we discovered that the unbound energy produced by the third synthesis actually provided the fuel for the first synthesis. The universal unconscious appeared as the remains of a multitude of dissolved consciousnesses that had reached a high point of self-annihilation. Similarly, ER can't become incoherent and dissolve the idea of repetition without taking the thinker and the whole world along with it. It's a thought that dissolves all the forms of identity, including its own. God, self, and world -- natural law, conceptualizer of that law, and universe -- all dissolve together.
The eternal return is a force of affirmation, but it affirms everything of the multiple, everything of the different, everything of chance except what subordinates them to the One, to the Same, to necessity, everything except the One, the Same and the Necessary. It is said that the One subjugated the multiple once and for all. But is this not the face of death? And does not the other face cause to die in turn, once and for all, everything which operates once and for all? If there is an essential relation between eternal return and death, it is because it promises and implies 'once and for all' the death of that which is one. If there is an essential relation with the future, it is because the future is the deployment and explication of the multiple, of the different and of the fortuitous, for themselves and 'for all times'.
It's brain-busting to try and conceive of the "completed but unlimited" fractal world that the eternal return implies. Nested within 'the' world are an infinite number of other worlds, and nested within those are ... 'The' world is this monstrous and completely unique infinite totality that nevertheless repeats itself internally. The crazy structure is necessitated by thinking about the world as a whole, including about how you could possibly be thinking about it as a whole. The only thing that is not present in this infinite unfurling is any stopping point, any natural unity, or pre-established harmony, or fixed set of laws that remain outside it. It's everything -- a cosmos as chaos, where anything can happen because everything does happen.
We began our exploration of whether the universe repeats by separating the question into the application of a fixed set of external laws to some initial condition. The laws were presumed to always be the same, but the condition might (or might not) be chosen randomly every time we run the experiment of the world, so to speak. This gave the world a single fixed probability distribution of possible outcomes once and for all. But chimp science seems to pose an insoluble metaphysical problem for this way of looking at the world. Instead, we're forced to imagine a unique world where the laws themselves change with each 'repetition', as if there were another stage of randomness in which certain laws were chosen, and from there an initial condition selected. But this goes on ad infinitum, because what distribution of possible laws were the particular laws selected from? Etc ... What we end up trying to conceive is a sort of total chance, chance multiplied by itself again and again, raised to a higher power. Hopefully this thought illuminates Deleuze's reflections on the infinite game that the eternal return represents.
It is claimed that man does not know how to play: this is because, even when he is given a situation of chance or multiplicity, he understands his affirmations as destined to impose limits upon it, his decisions as destined to ward off its effects, his reproductions as destined to bring about the return of the same, given a winning hypothesis. This is precisely a losing game, one in which we risk losing as much as winning because we do not affirm the all of chance: the pre-established character of the rule which fragments has as its correlate the condition by default in the player, who never knows which fragment will emerge. The system of the future, by contrast, must be called a divine game, since there is no pre-existing rule, since the game bears already upon its own rules and since the child-player can only win, all of chance being affirmed each time and for all times. Not restrictive or limiting affirmations, but affirmations coextensive with the questions posed and with the decisions from which these emanate: such a game entails the repetition of the necessarily winning move, since it wins by embracing all possible combinations and rules in the system of its own return.
Finally, Deleuze is right that absolutely no one has done better than Borges in imagining this dizzying world of total chance. I won't re-quote the Borges on pg. 116 here because we ought to just re-read the two stories that the quote mashes up. The Lottery of Babylon, and The Garden of the Forking Paths.