Reading his philosophical history, an idea, Bertrand Russell informs, of the Stoics in the person of Zeno is that of eternal recurrence, an unusual idea I've seen elsewhere described by Nietzsche; the idea of Zeno's being that at a cycle of life ends in a universal conflagration, whereupon the whole cycle repeats itself. "Everything that happens has happened before, and will happen again, not once, but countless times." I'd presumed in Nietzsche's case, the only awareness I had previously of this notion, that it was some way of trying to get people to focus on the importance of the life being lived now, and to hopefully act as an imaginative deterrent to condemning oneself to a conceived infinity of immersion in a life unworthy of oneself, so to speak. But perhaps they really are serious about this odd idea, and so to take a bit of a look at it.
Eternal recurrence purports to say that everything happening now has already happened. So this has all already happened. The key word here is 'this'. This has happened before. Well if something else has happened before, and so necessarily distinct from this, then this something else has to be as said something else, and so cannot be this. The very fact that you are talking of it as happening before means you are talking of something other than this. 'This' can only be itself exactly as it is, right now. To talk of something happening before is to introduce a 'that', i.e. something distinct from this, whereas this can only be precisely this. Also it makes no sense to talk of phenomena as if abstracted from the time element; and since one cannot effect this abstraction, then it makes no sense to talk of the same phenomena, including intrinsically the inseparable time element, occurring in a separate time.
When one examines the concept as the linguistic construct that it is, eternal recurrence of identical phenomena can only imply the present simply in its present sense. To try to talk of this recurring of previous time as though this something recurring is other than this here right now would be to defeat the concept, as then the phenomena would be different rather than identical as is necessary to the concept. And so for what is recurring to be, as is necessary to the logic, identical with what is happening, then it would have to be precisely and inseparably this, and so being precisely this cannot have occurred at a previous time, and so the recurring idea is rendered senseless. For something to be exactly like itself without the tiniest deviation is not actually to be like itself, but to actually be itself.
With eternal recurrence one is talking, or rather trying but failing to talk, of two different phenomena - something and its later exact recurrence - and progressively onwards to an endless number of different phenomena, and so these separate phenomena are not 'this'- what is now which can merely be one unseparated phenomenon - but separate phenomena, and being separate cannot be the same. Its apparent existence as an idea owes itself to people not looking closely at its actual logic, itself as meaningful language, and then seeing that the logic or the meaningfulness of the language quickly falls apart.
In essence, the word this implies something different than the word that, while the faulty language construct of eternal recurrence tries to assert this and that as referring to the same phenomena. The logic of eternal recurrence all merely leaves us with the present as is.
And so eternal recurrence is just a mad notion possessed of a certain kind of infuriating but senseless artistic merit.