What exists? On the one hand, this seems like the kind of naval-gazing question that provokes derision and mockery from those more interested in practical matters. I exist, you exist, this blog post exists. It's self-evident, right? Of course one could simply presume that everything they experience is nothing but a dream or illusion and that only they, themselves, actually exist. But solipsism is a useless philosophy when dealing with the IRS, say, or anyone else for that matter. So it may seem to be a silly question to ask.
On the other hand, when one delves into it more than superficially, defining existence turns out to be about as complex as defining consciousness. Putting solipsistic arguments aside, there are some things that quite obviously exist. But then there are grey areas. In my recent blog post on consciousness, I mentioned that there was a good deal of overlap between the nature of consciousness and the nature of existence. Giulio Tononi, as I pointed out, believes that there are gradations to existence that are a result of the causal power of something. I gave the example of a painting that is completed by a painter, but then the painter and painting are immediately engulfed in flames such that no record is left of the painting leaving us wondering if it ever existed in the first place.
At a certain level, it is absurd to think it didn't exist simply because no record of it was ever made. This is actually just a rehashing of Maxwell's demon; there is a record of it somewhere in a real universe because the act of painting it increased the entropy of the universe in some manner. A better question (and, truthfully, the real question I have about the painting) is, what became of the information associated with the aesthetic appreciation of that painting?
To put it another way, I can imagine many fanciful things that I know simply cannot exist because they violate the laws of physics: artificial gravitational fields in relatively small, non-rotating spacecraft, spacecraft that make sound in empty space, etc. While they may not be physically realizable, they nevertheless exist in my imagination which, as part of my mind, which very clearly exists. (If you read the article on consciousness, you may recall that this was Tononi's starting assumption about consciousness.)
In philosophical circles, this is known as ontological commitment and, as the name suggests, refers to a relation between a language and something that is proposed to be "extant" by that language, i.e. something that language says exists. It is generally understood that the "thing" that is proposed to exist does not necessarily have to be physical. One of the earliest and most influential formulations of ontological commitment was given by the philosopher W.V. Quine. What is interesting is that it centers around what can be stated in a formal language. In other words, it would seem to rule out the possibility of the existence of things that are "unspeakable," i.e. not representable in a formal manner. This is, of course, closely (though not perfectly) aligned with Heidegger's famous question, 'What is a thing?'
In recent decades, physicists have even begun to consider the issues of existence and "thingness." Chris Isham and Andreas Döring, for example, have even ventured so far as to discuss the latter directly in their work on topos theory in physics, something most physicists might be tempted to avoid, at least explicitly. So it is that FQXi convened an entire panel at this year's conference devoted to discussing the concept of existence.
While Tononi was not actually on the panel himself, he did, as I mentioned, address the issue when he discussed consciousness, equating levels of existence with degrees of causal power: maximal existence is possessed by things with maximal causal power. Though the concept of gradations of existence is missing, Rafael Bousso's theory of existence could be viewed as philosophically kin to Tononi's. Bousso makes the claim that the only thing that exists is a particular causal "patch" in spacetime (this is somewhat similar to the concept of a causal "diamond"). It is his view that everything that we can measure is in a particular causal patch and it is meaningless to consider anything else. It might be tempting to think that Bousso's approach is just a restatement of the hard-line operationalist view that would deny the existence of the moon if no one is looking at it, but I think that would be a mischaracterization. What he is really saying is that it is meaningless to talk about the existence of things that we have no hope of ever measuring. For example, he emphasized that this rules out the existence of a typical multiverse since it isn't contained within our causal diamond (no word on what his theory might say about Wiseman's many-interacting worlds hypothesis). The causal patch does contain many possible histories which, I suppose, might make it compatible with some consistent history theories of quantum mechanics. But the causal patch, which appears to be Bousso's only bound on measurability, is fairly limiting. For example, it conveniently does not rule out the non-measurable aspects of string theory (of which he is a proponent). The fact remains that not all limitations on measurability are necessarily due to the dynamics of space and time.
Some of the questions I have already raised are indicative of the types of problems that the concept of a causal patch does not address. For example, Jenann Ismael asked how we can meaningfully talk about evolving interactions between the mind and the world if the mind is in the world? The more general formulation of this question might be to ask how we can meaningfully talk about interactions between a sub-system with a larger system of which it is a part. But then, as Steve Giddings pointed out, how do we properly define a sub-system?
When polled on the concept of existence, the panel offered a wide array of views, from Carlo Rovelli's musings about the "existence" of Hamlet, to Laura Mersini-Houghton's assertion that existence requires an observer. Ismael explicitly mentioned Quine by name, though said her views are a somewhat modified version of his arguments about ontological commitment. Bousso took a slightly different tack when pressed on the topic and said that, ultimately, what matters are the fundamental, base axioms from which everything else can be derived. In my notes, I wrote "I'm surprised to find myself agreeing with Rafael" on this point. But in hindsight I'm not sure why I wrote that since every attempt to axiomatically derive physics has, to date, failed. I wrote my PhD thesis on one such failed attempt (Eddington's). So I suppose I will fall back on Tononi's position: I know I exist. Perhaps the rest of you can be derived, but perhaps not.