During a recent online discussion I discovered, somewhat to my surprise, that there is no general agreement on what the word “exist” means. Everyone has an intuitive understanding of it but when it comes to an explicit definition of the word there is no consensus, and indeed philosophers have written a vast literature on the topic of ontology, or what exists.
Dictionaries don’t really help; for example Oxford Dictionaries gives a nicely circular set of definitions:
Exist: 1. have objective reality or being
Reality: 1. the state of things as they actually exist
Being: 1. existence.
Of course physicists have a perfectly good operational definition: something exists if it is capable of making a detector go ping. Try arguing that, however, and you’re immediately accused of materialism, physicalism, scientism, being blind to possibilities beyond a very narrow world-view, and a host of similar sins (I plead guilty to at least the first three).
Many want to believe in the existence of things that are non-physical and do not make physicists’ detectors go ping. Such people avoid any explicit definition of what “exists”, preferring to rely on human intuition. I’m often puzzled by the weight given to intuition in philosophy. Yes, it is a guide, but we know that human intuition is horribly suspect and fallible, and unreliable unless corroborated by evidence. As H. Allen Orr says: “a scientific education is, to a considerable extent, an exercise in taming the authority of one’s intuition”.
I thus thought it worthwhile to take the physicists’ “ping” definition and see how it compares with philosophers’ ideas about ontology. Refining the definition a little, I arrived at:
Something “exists” if there is a potential chain of causal links between it and our sense data.
This is an empiricist’s stance, placing our own sense data at the root of our knowledge of the world. Even a pinging detector is useless unless we get to know about it. To adapt Descartes: “I feel therefore I am, and things that cause me to feel are”.
Of course we hardly ever experience anything directly, since even when looking directly at something we are experiencing the photons bouncing off it, not the thing itself. Hence we know about something only because it has affected a thing which affected another thing which affected something else … and ended up affecting sense data. Hence the “chain of causal links” in my definition.
The causal connections need only be between adjacent links in the chain. There might not be a causal connection between the ends of the chain (between the thing itself and our sense data) since, owing to the finite speed of sending a signal along the chain, the signal might not be able to travel that far.
For example, our universe has an “observable horizon” beyond which we can not know about, since even light-speed signals will not have had time to reach us given the finite age of the universe. Presumably, though, stuff still “exists” out there, even if we can’t see it (horrible consequences follow if that isn’t the case, including a violation of the Cosmological Principle, and the fact that material might go out of existence or come into existence as it crossed our observable horizon, as if we mattered).
The word “potential” is included in the definition since many possible causal effects will not, in practice, be realised. Lastly, I should probably remove the word “is” from the definition of “exists” and make the definition iterative: Start with the sense data; Thing 1 potentially affecting it then “exists”; Thing 2 potentially affecting Thing 1 then exists; and so on along the chain. Such a chain is, of course, our mechanism for discovering the world around us.
The above is a definition of whether something “exists”, not of whether we know that it exists. Obviously we can’t know about anything existing beyond the observable horizon, and we can only know about what has produced actual causal effects, not what just has the potential to do so.
Do composite objects, such as crystals or fruits or stars “exist”? Yes, these exist, since they are patterns of material, and detectors can be designed to go ping about particular patterns of material. Indeed, this is always the case, since nearly everything is a pattern of lower-level stuff (molecules being patterns of atoms, which are patterns of nuclei and electrons, and the nuclei are patterns of nucleons, which are patterns of quarks and gluons, which … well we’re getting beyond established physics to go further).
What about ideas and thoughts and emotions, do they “exist”? Does pain exist? Yes, since these are manifest as patterns of matter (patterns of electrical and chemical impulses in our brains) and, as above, patterns of matter can quite readily make detectors go ping (an fMRI scanner can read thoughts, though only very crudely as yet).
What about mathematical “objects”, such as an idealised perfect circle, or the number 7, do they exist? Not in their own right, but conceptions of these do exist, as ideas in our brains.
What about properties or attributes, such as the colour red? Does the colour red “exist”? (This is the philosophical Problem of Universals.) Things coloured red certainly do, and the conception of “red” also does in our brains. But “red” doesn’t exist in its own right. This is the philosophical stance of conceptualism, with a long history back to at least William of Ockham.
What about classes of objects, such as the collection of all members of a species, does the class, the species, exist in its own right? I’d say, no, though the concepts certain do, again as patterns in our mind.
Now, suppose one proposed a new type of non-interacting physical particle which interacts with other such particles but has no interaction at all with the known things in the universe. Could that particle be said to “exist”? A similar postulate is that there are faeries which can never be seen or heard or interacted with at all.
One response is to shrug and declare the question uninteresting as unfalsifiable and literally of no consequences. Under the above definition, though, they clearly (ex hypothesi) don’t “exist”.
How about causally disconnected parallel universes in a proposed multiverse? Again, ex hypothesi, causally disconnected parallel universes would not “exist” (though one could invent the concept that they might “para-exist”, along with the unseen faeries and the non-interacting particles). Note, by the way, that some multiverse models propose multiple connected universes, and these universes would exist in the same way that matter beyond the cosmological horizon does.
What about the multiple worlds in Everett’s interpretation of quantum mechanics? Since these go causally disconnected immediately on splitting they would not “exist”.
So that is how a philosophically naive physicist might see “existence”. So far I’m not aware of significant flaws in this way of defining “existence”, nor of any better general definition of the word — but if there are I’d be interested to know about it. Of course those not into scientism would have a fit of the vapours over a definition that is close to: “If we can’t see it then it doesn’t exist”.