Does McTaggert or anyone say this about the A series? Would it be nonsense to do so?
The past exists now [because now is the end of the past] but no time always exists now: so the past that exists now won't always be the past
I would like to suggest that the notions of past, present, and future are all nonsensical.
A complete scientific description of the universe would not tell us when now is. The "present moment" cannot be endowed with any special property that is not shared by any other temporal moment.
Thus, the concept of now appears to be a purely subjective one, and one which is obviously very important to the way the world appears to us. Yet we never seem to experience the present moment. The processing of sensory information takes time. Our perception of the world around us must travel through our nervous system along the various neural pathways to our brain. There it must be integrated with the disparate sensory information we are receiving to form a coherent representation of our experience. Yet once this process is complete and the information is cognitively available to us the moment has passed.
I'm not sure what the statement means or what its context is, but it seems consistent with the A-series definition of time, though highly dependent on our interpretations of "the" and "always."
In the tensed view of time, the past is not static, of course. It continually changes as things become more distant from the reference horizon of the present...becoming "more past." And in the "block universe" A-series, the quantity of things that are "past" always increases.
Thus, the "past that exists now" will not always be "the past," in the sense of the same past. It cannot be, obviously. Can it become "not past" in the sense of becoming the present or future? Not sure.
But I don't think that is a characteristic of McTaggert's A-series or of a Hericlitean stream, unless we assume a finite material universe and a Hericlitean recycling "fountain." But I suppose it is more likely in an A-series than in a B-series, where it would be ruled out, I believe.
But again, not sure what is meant by "won't always be the past." Both "always" and a singular thing called "the past" invite all sorts of curvaceous, paradoxical turns. I don't think there is proper license in A-series time for an "always" that is not synonymous with "now."
McTaggart apparently was something of a Hegelian, so probably well read up on ancient ontology - Hegel does mentions this specifically in the introduction to his Logic.
The definition of now - the present moment - for Aristotle, is:
The now is what holds time together, as I have said, since it makes past and future time a continuous whole, and it is a limit of time, in the sense that it is the beginning of one time, and the end of another.
ie this moment, right now, is the end of the past, and the beginning of the future - thus, also it's limit.
Notably, he says it divides time potentially, and not actually; as it must be otherwise the world wouldn't be causally connected.
But it would be well to source a quote...
It was brought to my attention that now is not unequivocally the past because now is also the beginning of the future. So it doesn't work out. But I kept plugging it so will post that for any comment.
But also the end of the past is not unequivocally in the past [it can be now] and so the end of the past can always have the same non trivial [perhaps there is no end of the past: so it isn't now] A time. That means there are tensed expressions [the end of the past is now] that are true at every A time: that there are A facts [about the end of the past] with every A time. And the end of the past is not more primitive than the past [which then also has every A time]: McTaggert is right