Paul Almond suggests a kind of theory of relativity applied to existence (section 3.1):

It makes sense to view reality in terms of an observer-centred world, because the only things of which you have direct knowledge are your basic perceptions – both inner and outer – at any instant. Anything else that you know – including your knowledge of the past or future – can only be inferred from these perceptions.

We are not trying to establish some silly idea here that things, including other people, only exist when you observe them, that they only start existing when you start observing them, and that they cease existing when you stop observing them. Rather, it means that anything that exists can only be coherently described as existing somewhere in your observer-centred world. There can still be lots of things that you do not know about. You do not know everything about your observer-centred world, and you can meaningfully talk about the possibility or probability that some particular thing exists. In saying this, you are talking about what may be “out there” somewhere in your observer-centred world. You are talking about the form that your observer-centred world may take, and there is nothing to prevent you from considering different forms that it may take. It would, therefore, be a straw man argument to suggest that we are saying that things only exist when observed by a conscious observer.

As an example, suppose you wonder if, right now, there is an alien spaceship in orbit around Proxima Centauri, a nearby star. What we have said does not make it invalid at all for you to speculate about such a thing, or even to try to put a probability on it if you are so inclined. The point is that any speculation you make, or any probability calculations you try to perform, are about what your observer-centred world might be like.

This view is reasonable because to say that anything exists in a way that cannot be understood in observer-centred world terms is incoherent. If you say something exists you are saying it fits into your “world view”. It must relate to all the other things that you think exist or that you might in principle say exist if you knew enough. Something might exist beyond the horizon in your observer-centred world – in the part that you do not know about – but if something is supposed to exist outside your observer-centred world completely, where would it be? (Here we mean “where” in a more general “ontological” sense.)

As an analogy, this is somewhat similar to the way that relativity deals with velocities. Special relativity says that the concept of “absolute velocity” is incoherent, and that the concept of “velocity” only makes sense in some frame of reference. Likewise, we are saying here that the concept of “existence” only makes sense in the same kind of way. None of this means that consciousness must exist. It is simply saying that it is meaningless to talk about reality in non-observer-centred world terms. It is still legitimate to ask for an explanation of your own existence. It simply means that such an explanation must lie “out there” in your observer-centred world.

This seems right, more or less, but it could be explained more clearly. In the first place Almond is referring to the fact that we see the world as though it existed around us a center, a concept that we have discussed on various past occasions. But in particular he is insisting that in order to say that anything exists at all, we have to place it in some relation to ourselves. In a way this is obvious, because we are the ones who are saying that it exists. If we say that the past or the future do not exist, for example, we are saying this because they do not exist together with us in time. On the other hand, if we speak of “past existence” or “future existence,” we are placing things in a temporal relationship with ourselves. Likewise, if someone asserts the existence of a multiverse, it might not be necessary to say that every part of it has a spatial relationship with the one asserting this, but there must be various relationships. Perhaps the parts of the multiverse have broken off from an earlier universe, or at any rate they all have a common cause. Similarly, if someone asserts the existence of immaterial beings such as angels, they might not have a spatial relationship with the speaker, but they would have to have some relation in order to exist, such as the power to affect the world or be affected by it, and so on. Almond is speaking of this sort of thing when he says, “but if something is supposed to exist outside your observer-centred world completely, where would it be?”

Almond is particularly concerned to establish that he is not asserting the necessary existence of observers, or that a thing cannot exist without being observed. This is mostly a distraction. It is true that this does not follow from his account, but it would be better to explain the theory in a more general way which makes this point clear. A similar mistake is sometimes made regarding special relativity or quantum mechanics. Einstein holds that velocity is necessarily relative to a reference frame, so some interpret this to mean that it is necessarily relative to a conscious observer, and a similar mistake can be made regarding quantum mechanics. But a reference frame is not necessarily conscious. So one body can have a velocity relative to another body, even without anyone observing this.

In a similar way, a reasonable generalization of Almond’s point would be to say that the existence of a thing is relative to a reference frame, which may or may not include an observer. As we are observers in fact, we observe things existing relative to our own reference frame, just as we observe the velocity of objects relative to our own reference frame. But just as one body can have a velocity relative to another, regardless of observers, so one thing can exist relative to another, regardless of observers.

It may be that the theory of special relativity is not merely an illustration here, but rather an instance of the fact that existence is relative to a reference frame. Consider two objects moving apart at 10 miles per hour. According to Einstein, neither one is moving absolutely speaking, but each is moving relative to the other. A typical philosophical objection would go like this: “Wait. One or both of them must be really moving. Because the distance between them is growing. The situation is changing. That doesn’t make sense unless one of them is changing in itself, absolutely, and before considering any relationships.”

But consider this. Currently there are both a calculator and a pen on my desk. Why are both of them there, rather than just one of them? It is easy to see that this fact is intrinsically relative, and cannot in any way be made into something absolute. They are both there because the calculator is with the pen, and because the pen is with the calculator. These cannot be absolute facts about the pen and the calculator – they are relationships to the other.

Now someone will respond: the fact that the calculator is there is an absolute fact. And the fact that the pen is there is an absolute fact. So even if the togetherness is a relationship, it is one that follows logically from the absolute facts. In a similar way, we will want to say that the 10 miles per hour relative motion should follow logically from absolute facts.

But this response just pushes the problem back one step. It only follows logically if the absolute facts about the pen and the calculator exist together. And this existence together is intrinsically relative: the pen is on the desk when the calculator is on the desk. And some thought about this will reveal that the relativity cannot possibly be removed, precisely because the relativity follows from the existence of more than one thing. “More than one thing exists” does not logically follow from any number of statements about individual things, because “more than one thing” is a missing term in those statements.

This is related to the error of Parmenides. Likewise, there is a clue here to the mystery of parts and wholes, but for now I will leave that point to the reader’s consideration.

Going back to the point about special relativity, insofar as “existence together” is intrinsically relative, it would make sense that “existing together spatially” would be an instance of such relative existence, and consequently that “moving apart spatially” would be a particular way of two bodies existing relative to each other. In this sense, the theory of special relativity does not seem to be merely an illustration, but an actual case of what we are talking about.