> What does non-physical existence even mean? This isn't an idle question

Much of modern life exists by convention only (by agreement among people) and has no physical existence. Bank balances, ownership of property, no-parking zones, national borders, having a college degree, the prestige level of a school, friendship, being a Seahawks fan, agreements, the law, political affiliation, religion, etc.

But all those things have an "ontology" (mode of existence) owing to being constituted by or supervening on things we know to exist (carbon, hydrogen, electrons, etc). These things are abstractions, but ultimately are grounded in physical existence either by the instantiating substance or substances that operate on the abstraction (e.g. brains). But those who argue for non-physical existence of things such as numbers are going further than this by saying that these non-physical entities are independently members of the constitutive fabric of the universe. This seems entirely unsupported and unnecessary.

The concept of existence seems to require conscious observers.

Sure, inasmuch as concepts require conscious minds. But surely the universe would exist if it had never spawned conscious observers. So I don't see that the question of existence begins and ends with conscious observers.

It has to begin and end with conscious observers, because they are the only ones that can consider the question. The universe does not need your answer, you do. The question quite literally ends when you do.

Why? Did universe not exist before the first life appeared?

The meaning of the word "exist" implies that it has some way to be detected by conscious observers.

Consider this: if I tell you there about a parallel universe you can never detect or discover, how does saying "BUT IT EXISTS" differ from saying "GOT YOU, IT DOESNT EXIST"?

I get your hypothetical example, however, will the universe cease to exist after the last conscious observer dies? For that observer, one minute before their death, would/should they suppose that universe will cease to exist or continue existing?

If we humans manage to wipe out ourselves next year in a nuclear holocaust, then is the continued existence of our planet conditional on some other conscious life arising in the future? Obviously it continues to exist even in your definition because they would be able to observe it, but how could such life arise if the universe doesn't exist?

I'd rather define "exist" as having an effect on our physical reality, which would obviously be detectable by "observers" in e.g. the quantum mechanic meaning of "observation" that has no relation to consciousness.

You're focusing too hard on the "existence" part and not hard enough on the "thing" part.

If humanity died tomorrow, nobody would be around to call the planet "Earth". Earth is merely a concept held in the minds of people. The concept consists of properties, only some of which involve the physical aspects. For example, lots of other planets are physically present, but only one we call Earth, because we live on it. Should we not live on it anymore, why would anyone bother to call it by a name.

In that sense, sure, Earth would cease to exist if we stopped living on it.

The problem with defining existence as only involving the physical is that it yields a world in which nobody can make sense of anything. You can't simply boil everything down like that and actually live in that world. The very second you started thinking, you'd start ascribing properties to objects that aren't merely physical, recreating the entire debate.

Before you consider existence, think about what "thing" means.

You said it yourself: "having an effect on our physical reality". It is a binary relation: something exists because we can experience it or deduce it from other things we experience.

In your hypothetical example, the existence a planet after it has eg exited our visible universe (because of the rapid expansion of the universe) is only relevant to us because we were able to deduce it from previous observations. Same with things in the past. So to answer your question, since we observe the Earth now, we can deduce its continued existence even after a cataclysm that would wipe out all life on Earth. We could be wrong in our extrapolation. Maybe the Earth would be hit by an asteroid later and things would be different in a way no one imagined. But, without conscious observers at that point, that state of affairs would be not unlike physics in that parallel universe. In that case, according to your definition, it wouldn't exist.

All of these are implemented in specific patterns that actually exist in reality and are detected by the brain.

> The word "exist" has a foundational importance in all of human intellectual activity.

Are you sure about that? I have a personal mental exercise / thought experiment where I try to iterate on my understanding of what it means to exist. But I wouldn't call that foundational. I maybe run through it once a month or so. Certainly it's attained a great deal of importance, but really, iterating on my understanding is never going to really get me anywhere.

Anything can be said to "exist". If you are thinking about it, it's currently existing in your mind as thought. If you're not thinking about it, but have thought about it in the past, then it exists in your memory. If you've never thought about it, then it's a potential that you could experience someday. It could also exist as a potential quantum state that nothing living could ever consider.

I'm reminded of the "library" where every possible grouping of characters is tied together with a math function and you can search for whatever you want. Considering whether everything ever written 'exists' or not just kinda lost its thunder to me. If everything meets some criteria, then it's the criteria that needs to be examined, not the everything. Archimedes wanted to move the world with his lever, but really, it's way more interesting to just move half.

You'd have to somehow escape your own mind before you could truly consider something that doesn't exist somewhere.

I don't think existence, in that sense, is as big of an idea as people seem to think. The way I usually use it is to get at the fundamental quality of being human. I think about agency, goals, limitations, purpose. These things seem way more interesting than trying to drive at the fundamentals of everything.

What it means for me to exist is way cooler to think about than what it means for my bed to exist.

I'm perfectly fine with this deflationary concept of existence, and leave the "foundational importance" to particular kinds of existence that actually warrant it. Once its articulated like this I have no problem. But my problem is with those who want to speak of existence of abstract objects in terms that claim such entities are independently members of the constitutive fabric of the universe. I just have no idea what this kind of existence means.

Your definition of physical existence above is not circular in some sense, it is circular in any sense since you define physical existence in terms of interacting with other physical existence. Just to test your definition, are colour or sound physically existent in your definition?

It's "in some sense" because we're defining the physical in terms of causal closure, rather than explaining the true nature of the physical (i.e. what physical matter supervenes on).

>are colour or sound physically existent in your definition?

Hard to answer without first having some kind of theory of mind. If we take the mind to be solely comprised of physical matter then mental states are particular organizations of that matter. Color and sound being mental states/percepts, they would be particular abstractions of physical matter. The abstraction itself would have no physical existence.

I am not sure I get your definition of causal closure. Would "black is what looks the same colour when put next to a black item" qualify as a causal closure, and if yes how can that be a non purely self referential definition in any sense? On colour and sound, how do you separate between the physical representation of the abstraction and the abstraction itself? You seem to be approaching a causal closure of non-physical existence, though one I am not sure is right.

The purpose of a definition is to pick out only those instances of a set you're attempting to refer to. The set of objects subject to causal influence by members of the set is well-defined and unique. Your example with black doesn't work because there are multiple non-intersecting sets that satisfy the critera, so we need some further conceptual machinery to distinguish them.

>You seem to be approaching a causal closure of non-physical existence, though one I am not sure is right.

I'm not sure I follow. To be clear, the "causal closure" was referring to the fact that the physical is subject to causation, rather than a generic term describing a set of self-similar items.

> They're both ideas, but we mean something very different.

are you sure about that?. words are a social convention adopted to provide a vehicle for transferring meanings from one mind to another. what are the meanings underlying the words though? why do you agree that the conventional word "idea" can be used to signify both meanings, but then claim that the meaning is somehow very different.

>why do you agree that the conventional word "idea" can be used to signify both meanings, but then claim that the meaning is somehow very different.

Well I can only go by how the words are used. But when we're speaking rigorously, such ambiguities matter. The issue is that there are multiple ways these words are used. But when we try to use the casual meaning in more rigorous contexts, the discrepancies are important. When it comes to existence, people are purporting that there is an existence completely unlike physical existence, yet retains its centrality in our conception of the world and its constituents. I'm looking for clarity here. If we can't flesh out the nature of non-physical existence, we should refrain from using the term "exists" in those contexts.

ambiguity is an inescapable consequence of systems of communication based on social convention. That's really the fundamental reason why physicists have such a strong preference for mathematics. The convention there is of logic and notation rather than social preference and personal idiosyncrasy as with natural language.

but we don't have the means to use a rigorous system of communication when discussing consciousness. there is no calculus of mind. its not even clear that there could be, in principle.

so we need to re-calibrate our notion of rigor and precision to deal with the domain we're working in here. we shouldn't just wring our hands and lament that our communication tools fail us. we can still use them, we simply need to be more disciplined and avoid devolving into arguments about language semantics and conventions and try to focus on the actual content, ambiguous as it may seem.

here's a simple working definition of "existence" that may be ambiguous but should hopefully not be controversial. a thing exists if it is experienced by a conscious mind. this includes material things which are experienced via the conventional senses (sight, hearing, etc.). it also includes immaterial things, such as thoughts and ideas, which are experienced internally within a conscious mind.

this definition does not resolve the ambiguity regarding the point-of-origin of immaterial things, but that is not particularly relevant in my opinion. we don't need to answer that question in order to engage with the material, so let's just leave it unanswered.

> What does non physical existence even mean?

It's a faulty concept. Everything is physical. But I think it refers to non-material aspects of the physical, especially patterns of cause and effect.