What does it mean to exist? I obviously can’t answer that ― no one can. But permit me, if you will, a perspective. I think existence can be subdivided into genres of existence, and not everything that is ‘true’ necessarily exists. There is the most obvious kind of existence, which is a physical presence in this universe; there is a more abstract existence, where a shared reality motivates people to action, but that reality is known to be a fiction; then there is a completely unknowable “existence”.
The obvious type of existence still has some interesting comments to be made about it. Although they are not the main point of this post, it is interesting to point to a few thoughts I have on the issue. There is a basic physical universe we observe, but the physical universe extends all the way down to quantum and all the way up to the relativistic. Those two ‘modes’ are not verifiable by traditional empiricism (the 5 senses); instead, they are verified through models of understanding and equipment that extends our sense. Empiricism, as a philosophy, has extended to include measurements from equipment: energy so faint our eyes could not see it, from quanta and from distant stars; heat so hot our bodies would never survive the sensation. This is the foundations of Model Dependent Realism: a model of understanding the physical universe that permits devices to extend our senses, to give further progressive models of reality.
This means that quantum particles can only be said to exist in this universe as a consequence of Model Dependent Realism, which is to say people have a firm understanding of the macro-world and the sensors and measuring devices they have built and what their readouts and the maths actually mean.
There are things that are true about this universe which do not exist, and that is why Physicalism (or even Materialism) are not adequate models of all that is “true”. I would argue that ethical statements fall under this category: about that which is true, but not that which exists. For example, there are wrong behaviours to practice if one’s goal is rational discussion. Many of these have been written down as “fallacies”. The difference between these is true, even if it doesn’t ‘exist’, per se. It is a valid description of how rational conversations happen, but it doesn’t refer to things that exist.
Immediately from the physical existence, there is emergent existence. These are properties that exist as a result of other things. For example, “blue” is not out there in the universe, wavelengths of energy are; “blue” is what happens when the right wavelengths hit a nervous system in the right way. Similarly, a sport emerges from players with an intent, in the same place, bound by agreed rules. Sports don’t physically exist, they emerge. (That is not to say they can’t be described in rational terms with reference to strategy and rules.)
But, take an entity like Harry Potter: does Harry Potter exist? No, the physical body does not exist in this universe. I don’t see that a defence could be make that Harry Potter exists in either of the senses described already in this post. However, Harry Potter is a motivating force, that compels people to cosplay and buy merchandise and draw fan fiction and buy the films. You could say the idea of Harry Potter exists, but then definition of ‘exists’ is stretched slightly further again, as the idea is shared and relates to an agreed realty. Plus, an idea is a personal thing, and yet Harry Potter is a shared concept. Which leads me to ponder whether it is defensible to consider a second type of existence: in conceptual reality. The ideas which compel people to action can be said to exist in this conceptual reality.
This may seem like a stretch, but consider for a moment that everything, eventually, is part of this conceptual reality. I have only had conversation like this in real life with people who I have a conception of as being interested in and able to have such a conversation. I could be mistaken, but it’s the conception I have. My concept of reality is what motivates me. Everything I think I know about reality is actually a conceptual reality. Even useful heuristics that teachers use to support understanding are a distant model compared with nuanced and accurate understanding. Don’t get me wrong, I see the distinction between my concept of reality, which is a model of the universe bound by the empirical evidence and testimony from dedicated experts, and the conceptual reality, which is a motivating fiction; these are at clear ends of a spectrum. But I do think acknowledging this blurs the line between physical and conceptual existence.
In this context, there are clearly ways to argue that a God exists. God is a motivating idea to many people, just like Harry Potter (but to more people and to a more encompassing level). God even exists in people’s conception of the universe. But, that point comes with caveats. Even if you accept my idea of a spectrum between an existing universe and a motivating but fictional concept, each point on that spectrum requires different rules of discussion. The closer one is to the ‘actually exists in reality’ end of the spectrum when they make a claim, the more heavily defended the claim should be by empirical evidence. The further to the it’s a fiction I like to arrange the project of my life around end one is, the more the discussion deserves a moral and ethical slant. But then one has to be careful about where on that spectrum they believe their claim lands. Harry Potter exists insofar that some people to organise themselves around the idea, but that is not the claim that they meet Harry Potter.
Which takes us slightly deeper into the the question of whether a God exists. Although I may concede that God exists in a conceptual reality, that is not what most people claim: people seem to claim that God exists outside of this reality, but exists in the same sense (though not bound by the same limitations) as my laptop. That claim requires evidence. If you think God is as real as the Great Pyramids, then you should engage in the discussion using similar rules: empirical evidence, rational discussion and reasoned doubt. Defences of God’s existence which talk about “God is a journey” or “God is how we understand humanity” or even “God grounds morality”… these place the claim of God’s existence at the fiction people organise their lives around end of the spectrum. The defence you try to use places the claim on the spectrum, and the validity or your argument is how it rises or falls.