this leaves the proposal of God's existence to be a possibility
That seems to be enough for you to believe that a god actually exists. It's not for the skeptic. Possibility is only of interest because the subset of things that are actual or can become actual resides there. Logic demands that that which can be shown to be actual is treated as actual, and that which merely can't be ruled out not be treated as actual.
why would anyone adopt the presumption that God/gods exists, or that God/gods do not exist
I don't know, but you seem to have done it.
Self-identifying atheists do not define atheism.
Sure we do. Theists' nomenclature for unbelief is inadequate. It doesn't reflect how most atheists think or see themselves. The definition you use is inadequate for me, because it would exclude me from atheism simply because I don't also declare gods nonexistent. That just doesn't work for me or for countless others. Look at the problems it's causing you here in this thread. You don't understand what any agnostic atheist is telling you because you can't conceptualize being atheist and agnostic at the same time. That is a result of your error that one can be one or the other but not both at the same time.
Most people are agnostic, and most atheists admit to being agnostic. So the question is why don't they simply remain agnostic.
Here's where you definition of atheist fails you. You simply cannot conceive of the fact that we can remain agnostic as atheists, and in fact, most of us are. And until you assimilate this simple fact, you will remain confused.
How does one operate under the assumption that there is no God as opposed to operating under the realization that they don't know if there is a God or not? What would be the actual difference? And if one has already accepted that they do not know if God exists or not, why would they then choose to presume that there is not?
The problem is that there are three positions possible on gods, but only two ways to behave. There are those who say that gods exist, those who say they do not, and those who remain agnostic (3). One can live as if there is a god or not (2). How shall we map the 3 onto the 2?
Those who believe that gods exist are naturally theists, and those who say gods do no exist are naturally atheists, but that leaves the agnostics, who must choose one of these as well. If the agnostic is an experienced critical thinker, he chooses to live as if there is no god for the same reason he chooses to live as if there are no vampires or anything else people have proposed exists, but can't be demonstrated or disproved.
The default position is to not believe until there is a sound reason to do so. Why? So that we don't have to randomly guess which of these things to believe in, or worse, believe them all. Or maybe we should also cover our doorways in garlic in case there are vampires. And keep silver bullets handy in case werewolves actually exist.
And you call that attitude illogical. Your argument is that if you can choose to believe in gods or not, why not believe. That's illogical. And multiple people have told you that, all atheists. So, you wind up with a thread full of atheists all using the same reasoning to come to the same conclusion that atheism is the proper position for agnostics, and one guy who can't conceive of agnostic atheism and who thinks it's illogical not to believe something without sufficient evidence, and who has chosen theism.
What do you suppose is the actual message (meta-message) you are sending those people? That you're a logical thinker whose conclusions should be considered more carefully, which is presumable the message you'd like to be conveying, or that tortured thinking leads to unsound conclusions? Do you think that you are pulling them toward your way of thinking, or helping reassure them that they have thought things through properly and made the right choice rejecting theism.
This is the question: why do atheist go beyond agnosticism to presuming that no gods exist when they have already determined that they can't know if gods exist or not? I understand why theists do it. I am asking why atheists do it, logically. So far the only answer I'm getting is "lack of evidence" which is not at all logical. Yet atheists claim they are all and only about logic.
The answer has been given to you multiple times, but it has been impossible for you to understand it. You'll just have to remain agnostic (unknowing). The answer is not available to your mind. Something prevents you from understanding it.
I am agnostic about the existence of God/gods. I choose to trust in the idea that a God of my own understanding exists because I find that doing so works in a positive way for me whether God exists or not.
Great. But your needs are different from mine. I have suggested that you use the notion of God to help you with personal struggles, and I have no problem with that. If it helps keep you remain centered, if it helps you tame your demons, I'm all for it. The belief may have transformed your life for the better.
As I have told you in the past, I have no unmet needs that a god belief would benefit. I likened this to you having blurry vision, discovering corrective lenses, and shouting out to the world that everybody should wear a pair, unaware that there are people who see well without them, people who would not only not benefit from wearing glasses, but actually have their vision degraded by them. But you can't imagine that, and call people illogical for not wearing a pair. Then someone like me, a former Christian, tells you that he sees fine without glasses, and in fact, actually saw better after taking them off. Then you say that I must have been wearing them wrong, or I chose the wrong prescription and should try again. You just can't see why everybody wouldn't want glasses after they rescued your vision, and calling others illogical for preferring no glasses.
This is essentially where we are with theism. You say that it worked in a positive way for you, and wonder why everybody doesn't follow your lead.
justice is a "human creation", yet I assume you aren't presuming that justice does't exist. Or that it's existence has no effect in and on reality. So why have you chosen to assume so with 'God'?
The word justice refers to activities that can be observed and judged. It is a concept abstracted from concrete instances that have a particular quality in common. The word God has no known external referent. That is the difference between the two. I can see instances of justice and injustice, so I have no reason to question whether justice exists, and I have no difficulty referring to those real events using those words.
But God? Am I referring to anything real? I don't know. So, I believe justice exists but not God even though both are abstractions because I can experience one and not the other. I understand that you think both should be believed because they're both abstractions, but that's not a good way to decide what is true about the world. The first is evidence-based, the second faith, and faith isn't a path to truth, given that any error can be believed by faith.
Logic defines atheism. I would think atheists, who are constantly touting the importance of logic, would appreciate that.
Logic leads to atheism. It is the only sound position for a skeptical empiricist.
I'd say that choosing the definition of atheist that you have chosen was illogical, since it confounds your thinking and impairs communication. This entire thread wouldn't exist if you had defined atheism as atheists do, or at the least acknowledged in your writing that that is how they define themselves and what they mean when they tell you they are atheists, and adjusted accordingly. But you refuse, a choice that has led to a chaotic discussion that generated nothing for you. You haven't learned a thing about why atheists are atheists after over 20 pages of discussion because of that illogical choice.
As an illustration, I'm a contract bridge player. We call the heart and spade suits the majors, and some of us will open the bidding 1 No Trump with a five card major, while others will open in the major (1 Heart or 1 Spade) with five or more of them. There are arguments for each position, and problems with both, so there is no consensus among bridge players on the matter. I won't open a hand in No Trump with a five card major, but I can play with a partner who will as long as I understand what he means by 1NT - he might have five hearts or spades. And he can play with me knowing that I won't have more than four of either suit when I open 1NT.
Of course, we have to use a different set of responses to 1NT than one another, because we each need to ferret out whether we have eight or more of either of these suits in our combined hands. The convention that accomplished this is called
Stayman, and it is a 2 Club answer to 1NT. My partner will ask me whenever he has a 4-card major to see if I have four more, but when he bids 1NT, I have to use a different form of Stayman. I have to ask partner about his major suits if I have a 3-card major, since he may have five in that suit. My version of Stayman is called
Puppet Stayman. So, we can still communicate effectively even though neither of uses the same definition of 1NT, nor mean the same thing when we bid it, because we understand what the other means, and adapt our responses accordingly. Now, that's logical.
Nobody here is asking you to change your definition of atheist, just to try to understand ours, and when dealing with somebody whose definition is not yours, adapting your responses to reflect that you understand them in order to communicate. Use Stayman (metaphorically, of course - a 2 club response to any post here would be indecipherable) when dealing with somebody who shares your definition, and switch to Puppet Stayman when dealing with those who mean something other than you do when they call themselves atheists.
Theism - God/gods exist.
Atheism - the antithetical - either can't decide or don't exist.
Since the undecided option is already called agnosticism, that leaves the don't exist option alone to be called atheism.
2 Clubs.