Otdfbt

I can conceive of an infinite past with a beginning. I can in fact represent this idea by simple diagram, part analogical, part symbolic. So, to me, this idea is a logical possibility.

I initially believed that nearly everyone should be able to do the same. Apparently, I was wrong. Many people object to it, vehemently, on the ground that the ordinary, conventional notion of an infinite past is that of a past which is infinite precisely because it has no beginning.

So, as the argument goes, the notion of an infinite past with a beginning would be a contradiction in terms, and this even though, unlike for example "bachelor", there is no dictionary definition of "infinite past", and there is therefore no dictionary definition of an infinite past as having no beginning.

As I understand it, our initial notion of the infinite came from our sense that time is going to continue and that, therefore, it is literally not finished, i.e. in-finite, or "not complete" as some people like to put it.

Still, since more than a century ago now, mathematicians have learnt to deal with the notion of actual infinite, i.e. the notion of an infinite that would be complete. This is not necessarily the same idea as that of an infinite with a limit, though.

As I understand it, the idea of an actual infinite came as a consequence of assuming the existence of a set containing an infinite number of elements. The number of elements is infinite but the set itself contains all of them and so is an "actual" infinite. This in itself doesn't imply that the set contains a greatest or smallest element but the set is thought of as containing the entirety of an infinity of elements, which seems to imply at least that the set is indeed a "complete", or an actual, infinity.

However, it seems to me that, for example, the interval of Real numbers [0, 1] is already conceived of as an actual infinite. It of course has a "beginning" and an "end". And I think of it as commensurable to an infinite past with a beginning, or even an infinite time with both a beginning and an end.

So, how would it be necessarily illogical to think of the past as both infinite and with a beginning?

Or why would it be somehow necessary that if the past is infinite, it has no beginning?

Aristotle said the past is infinite because, for any past time we can imagine an earlier one. Aristotle's arguments aside, this is what people mean when they speak of an infinite past: for any time x, there exists another time y such that y precedes x. Colloquially, "there is no first moment in time". If time has a beginning, it means that there is a time x, for which there is no time y such that y precedes x. Colloquially, "there is a first moment in time". This is a contradiction; so there cannot be both an infinite past (in the sense described above) and a first moment (a beginning).

Mauro ALLEGRANZA in his comments explains that there can be different ways something can be said to be "infinite", but in the context of philosophical arguments where an infinite past is discussed, it is probably the sense that I describe in my first paragraph.

EDIT: To expand a bit based on the comments, there are two other properties that time could possibly have, that would mean that time has an infinite number of moments, even if it did have a beginning (or even both a beginning and an end):

1. Time could be dense, which means that for any two times x, y, there is always a third time z, between them so that x precedes z, and z precedes y. If (the set of moments in) time is linearly ordered, then density implies that there are an infinite number of moments.




2. Time might be continuous or without "holes" in it, like the real number line.

Neither of these properties are what people usually mean when they say that the past is finite or infinite. Instead, they mean it like in my first paragraph. I believe when the OP is speaking about the past being "infinite", they are using it to mean something like either dense or continuous. This might be mere semantics, but once the multiple senses of "infinite" are disambiguated, the confusion and disagreement should disappear.

It depends on exactly what you mean by an infinite past.

Let's start by defining some terms so we can deal with this rigorously. Let t be an arbitrary time, and let t = 0 be the present. Any t < 0 is in the past; any t > 0 is in the future.

Let us suppose now that time has a beginning; we'll place it at t = a. There exist an infinite number of instants in time between a and 0. For example: -a/2, -a/4, -a/8, etc. For any natural number n, t = -a/(2^n) is a time after a but before 0. There are a countably infinite number of natural numbers, so there are a countably infinite number of such points. (And there are also an uncountably infinite number of points in that range that are not of the form -a/(2^n).

But we have an infinite number of elements only because we keep dividing it into smaller and smaller divisions. Suppose that instead of asking how many instants of time exist between the beginning and the present, we instead ask how many seconds there have been since the beginning of time. That number is decidedly finite.

In summary,
if there is a beginning of time, then there is a finite length of time between that beginning and the present, but we can divide that finite length into an infinite number of infinitesimal chunks. (Mathematically, anyway. Whether physics actually permits dividing it up that much is an open question.)

To answer this, we need to visit Hilbert's hotel.

It's an infinitely long corridor with an infinite number of rooms, and an infinite number of guests.

One day an extra guest turns up and wants a room. Hilbert can't send him down the corridor - it will take literally forever. So he asks all the guests to move one room down the corridor. The guest in room 1 moves into room 2, the guest in room 2 moves into room 3, and so on.

We can see that, while it was already an infinity, this does not mean that it can't be incremented by 1. An infinity does not necessarily equal another infinity.

What if an infinitely big coach turns up with an infinite number of guests? That's ok: you just ask all the existing guests to move into the next even-numbered rooms. The guest in 1 moves into 2, the guest in 2 moves into 4, the guest in 3 moves into 6, the guest in 4 moves into 8, and so on.

Now you have an infinity which is twice as big as it was before.

The point here: something can have a beginning and still be infinite. It can start at zero and go all the way up to a positive infinity. It doesn't have to start from a negative infinity, or even from zero. Can you start at 100 and count infinitely upward? Yes, of course you can. It's infinite as long as it doesn't have an end.

The stumbling block here is that, conventionally thought of, the past does have an end: the present. So there can be an infinite period of time with a beginning, but it has to stretch out into the future as well.

The term time, like temperature, implies basically two concepts: the concept addressed in physics, relativity, etc., and the feeling that we have of an ordered sequence of events. Let's start with the second.

Kant proposed that time and space were synthetic a priori knowledge, that means that our knowledge of space and time is not obtained from experience, but it is us that create them in order to have an understanding our environment. In such sense, time is just one of multiple ideas that allow our perception to be organized. Space is similar. Both are not really physical realities, but moreover mental features that our understanding of reality lie upon. Time and space are just possible due to our memory.

What would be the physical concept, in such case? Basically, mathematics and physics are a set of formalized rules that we can apply to perception, not to reality, and work perfectly! When we add or count 1+1=2 apples, all operations are performed in the realm of subjective perception, not in reality, and work absolutely fine. In reality, counting apples would be like counting clouds in a rainy dark night, since apples or clouds are just massive amounts of atomic interactions. In other words, any result depends on the subject, not on reality.

So, blending both ideas, your perception of time has a beginning: your first memories; and it has an end: now. But if we recur to the rules of mathematics and physics, it is clear that there seems not to be a beginning or an end, since due to mathematics we can always think of a previous moment to any beginning, or a future instant of any ending. But even physically, there seems to exist an instant where time appeared, instants after the big-bang, or an ending of the universe, where entropy will reach its maximum. Yes. But our perception can be contradictory with what mathematics and physics state, and that is completely normal. For example, that's the main issue with quantum mechanics. We cannot fully understand what physical and mathematical formulas state [1]. There's a lot of things out there that we cannot understand, even having the mathematical formulas that describe such reality.

That is the problem that you are confronting, and you're not the only one. And such incoherences of reality and perception (Kant's noumena and phenomena) are just part of the current challenges in philosophy and science.

[1] https://www.sciencenews.org/blog/context/tom%E2%80%99s-top-10-interpretations-quantum-mechanics