One sees arguments for the existence of non-physical entities such as God, qualia, Plato's forms, objective ethical truths, etc...

But what does it mean for something non-physical to exist?

It strikes me that the very definition of existence implies physical existence. Consider the following reasoning:

  • To prove that an entity exists, one either has to observe it directly, or logically derive its existence from something observable.
  • This can only happen if the entity has a causal effect on physical observables.
  • Anything that has a causal effect on the physical world is itself physical.

Per the Duhem-Quine thesis and the underdetermination of scientific theories, even empirical data can be explained by multiple theories, and even logic and math are empirical. Based on this, once we remove the constraint that a proposition has to correspond to observable facts, then it follows that there are no constraints at all on how to determine the truth of a proposition, and the existence of anything can eventually be proven. The concept of existence becomes arbitrary.

So my question is:

  1. What does it mean for something non-physical to exist?
  2. How can something exist without having a causal effect on the physical world?
  3. Is there any way to prove the existence of something non-physical?

  • Several commenters and answers have pointed out to me that abstract and mathematical concepts exist. Let me clarify the gist of my question: We consider concepts such as electromagnetic field, neutron, and gene to be physical entities. Yet they are as abstract as mathematical or social constructs. Why are the former considered physical while the latter aren't? The distinction seems arbitrary. Aren't they all manifestations or descriptions of physical phenomena?

  • A few of the comments and answers seem to misunderstand an important part of my premises: I am not starting from the premise that only that which is observable exists. I am postulating that either something has to be itself observable, or that it has to be something that can be inferred from observable facts, even if it itself is not observable.

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    "This can only happen if the entity has a causal effect on physical observables. " Why? This may be empirically true but not logically necessary. Yes, if we accept the premise that the brain needs to be affected for observation to take place, then the only way we can know about anything is if it has a physical effect. But it's not logically necessary for thoughts and experience to correspond to brain activity. It's logically possible there are thoughts that have no accompanying brain activity (hence no corresponding physical effect). May 7, 2016 at 4:16
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    @AlexanderSKing, you can't prove your own consciousness exists. Does that mean it doesn't? May 7, 2016 at 4:18
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    Does mathematics exist? Like that. May 7, 2016 at 14:15
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    "Anything that has a causal effect on the physical world is itself physical.". This would imply that time is physical, which is at least a questionable point.
    – Uriel
    May 7, 2016 at 19:01
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    so everything we observe is physical? are love or grace or hope or evil physical? do those things exist? are they physical? how about numbers? does 13 really exist? is it physical? May 8, 2016 at 2:36

13 Answers 13


Consider the case of numbers. Frege, for instance, argues that numbers exist as abstract objects. Here is an outline of his argument:

The language of mathematics purports to refer to and quantify over abstract mathematical objects. And a great number of mathematical theorems are true. But a sentence cannot be true unless its sub-expressions succeed in doing what they purport to do. So there exist abstract mathematical objects that these expressions refer to and quantify over. (SEP)

For example, it would be odd to accept the following

There are prime numbers between 10 and 20

as true, but to claim that there are no numbers. One could take another approach and say that it is true that there are numbers, but nevertheless that numbers do not exist.

But since the existential quantifier (∃) is used in expressing the above sentence, one would have to distinguish different sorts of existence, e.g. existence as expressed by ∃ and physical existence (or explain otherwise what is expressed by ∃). Note that this might amount to taking physical existence to be a property.

The main point is this. We talk about numbers, fictional characters, musical compositions, etc., all of which are abstract objects, and we can make true claims about them (e.g. 'Sherlock Holmes is more famous than any real detective'). These of course do not exist in the same sense that physical objects do, but it does not necessarily mean that they do not exist in any sense of existence.


You asked:

We consider concepts such as electromagnetic field, neutron, and gene to be physical entities. Yet they are as abstract as mathematical or social constructs. Why are the former considered physical while the latter aren't?

First off, anything physical is by most conceptions not abstract. The distinction is usually between abstract objects and concrete objects, where the physical objects are supposed to be some subset of the latter.

One common conception defines abstract objects as follows:

An object is abstract if and only if it is causally inefficacious. (SEP entry on Abstract Objects)

The entry continues:

Concrete objects, whether mental or physical, have causal powers; numbers and functions and the rest make nothing happen.

So the distinction is not arbitrary. This does, however, fit your statement that anything that has a causal effect on the physical world is itself physical.

  • Your first quote involves a non-sequitor. Then: accepting a claim as true need not entail ontological commitment. Achilles was a bad ass warrior. Superman can fly. i is the square root of -1. Hardcore intuitionism might deny that our ability to find primes implies that they "exist". The problem is natural language : we tend to take "x exists" as "x exists eternally, independently of us". If I build a house it exists, until somebody tears it down. Numbers are no different. Truth and existence are very different ideas.
    – user20153
    May 7, 2016 at 20:04
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    @mobileink A non-sequitor? That argument is logically valid. You can reject its premises, however, and claim that it's not sound. I'm not saying it's a position I hold myself but I'm referring to what philosophers have thought. (Also, please be more charitable in reading arguments.) Nowhere have I said that existence and truth are the same thing. But I am pointing out that there is something to be explained: how statements can be true while referring to nonphysical or nonexistent things.
    – E...
    May 7, 2016 at 20:11
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    Regarding primes specifically: a counter-argument might run like this. The proposition is ill-formed. Instead, consider sth like "We can construct the numbers from 10 to 20, and show that some of them are prime. If they can be said to "exist", it is only because we have made it so."
    – user20153
    May 7, 2016 at 20:11
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    Or to put is another way: existence is not a logical concept. "Existential quantification" is a misnomer -we quantify over stuff that is already assumed to exiat
    – user20153
    May 7, 2016 at 20:38
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    "An object is abstract if and only if it is causally inefficacious." I had a phil. of math prof who railed against SEP's presentation of abstract objects on an almost daily basis (he was a Platonist). The criticism is that our knowledge of abstract objects (such as knowing that 17 is prime), as well as anything we do with that knowledge, is obviously caused by the objects themselves, through our interacting with them. There's no point in positing abstract objects if they have no effect, since at that point everything can be explained without them. (I don't personally hold this position.)
    – Era
    May 10, 2016 at 18:08

What you are relying on is called causal or reliabilist theory of knowledge that goes back to Ramsey and Goldman. But all of its theses can be and were rejected by various philosophers. "Direct observation" does not have to be restricted to sense perceptions, and causal effect does not have to be physical-on-physical. Plato had pre-birth mindsight, Aristotle had passive and active intellect for receiving universals and other idealities, Spinoza had intellectual intuition (“the human Mind has an adequate knowledge of God's eternal and infinite essence”), and Husserl had categorical intuition (and was a realist in his early period). Penelope Maddy, a prominent analytic philosopher of mathematics, is a mathematical realist, and so is her mentor Burgess. She follows Gödel, who explicitly adopted Husserlian theory of mathematical intuition, see How does a realist account for causation between universals and particulars?

Ideal perception operating beside sense perception is supported by modern cognitive science at least on the phenomenological level. As a result, Aristotelian realism about ideal objects and abstractions generally is on the rise since 1990s, and is advanced especially by the Australian school (Armstrong, Bigelow, etc.), see Franklin's review:

"The objects may be of any kind, physical, mental or abstract. The mathematical statement does not refer to any properties of the objects, but only to patterning of the parts in the complex of the four objects. If that seems to us less a solid truth about the real world than the causation of flu by viruses, that may be simply due to our blindness about relations, or tendency to regard them as somehow less real than things and properties. But relations (for example, relations of equality between parts of a structure) are as real as colours or causes".

So far nominalism and conceptualism did not offer an account of universals and our acquisition of them that is clearly superior to the Aristotelian one, although Wittgenstein's family resemblance is perhaps the most promising alternative, see Is it possible to use Wittgenstein's family resemblance approach to universals to separate high art from commercial art?

This should dispel the mysteriousness surrounding the nature and supposed causal inertness of the non-physical. As for "anything that has a causal effect on the physical world is itself physical", it sounds to me question begging against idealists, theists, dualists, etc., who would most certainly reject this premise. Other than Spinoza, I can not think of a monotheist of any kind who thought that God was a physical being, and even his phrasing ("Extension is an attribute of God, or God is an extended thing") is open to interpretation. For analogy to the God-world relation think of a "simulated reality". Within it we have "physical" existence and causation imposed by the program, but the programmers and the hardware exist "non-physically", and exercise quite a different type of causal influence on the simulacra. Leibniz and Berkeley present metaphysics were the ideal "simulates" the physical in a similar manner. But one need not believe in God or simulated universe, they serve as models, like a Euclidean models of non-Euclidean geometry, which show that even a materialist has to admit that causally effective non-physical entities are logically and even "metaphysically" possible. It can only be argued that they do not exist as a matter of fact.

Qualia and mental states in general are a different issue due to the role of the subject/object rift. There is an analog of the causal inertness concern for them articulated in Kim's causal exclusion argument, see Is there a causal influence of the mental on the physical? But even with them there is a Grand Canyon wide ambiguity on what counts (or eventually will count) as "physical". I think Nagel in What is it Like to be a Bat? gives the sharpest perspective on the issue:

"I have not defined the term 'physical'. Obviously it does not apply just to what can be described by the concepts of contemporary physics, since we expect further developments. Some may think there is nothing to prevent mental phenomena from eventually being recognized as physical in their own right. But whatever else may be said of the physical, it has to be objective. So if our idea of the physical ever expands to include mental phenomena, it will have to assign them an objective character — whether or not this is done by analyzing them in terms of other phenomena already regarded as physical. It seems to me more likely, however, that mental-physical relations will eventually be expressed in a theory whose fundamental terms cannot be placed clearly in either category".

See Are there philosophies that call for things which are not mind nor matter? and How do modern dualists explain the mind-body interaction? for a broader perspective.

  • I think I failed to express the gist of my question: Once we accept the ideas from Quine's "Two Dogmas" what is left of mathematical realism? Logic itself is up for grabs, and based on a suitable background assumptions, all sorts of counter intuitive mathematical objects can be constructed, making their reality very tenuous May 9, 2016 at 16:16
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    @Alexander Quine's point is the use of words, he rejects intrinsic meanings and intuitions. So "odd number divisible by two" may very well be made true in some conceptual scheme that uses those five words, and Quine says explicitly that any statement can be maintained as true "come what may". But so what? It is not individual statements that face the "tribunal of experience", but the scheme as a whole. And in our scientific scheme, says Quine, sets and numbers are "indispensable". Therefore, they exist. Aristotle would turn in his grave from Quine's "existence" and his "realism".
    – Conifold
    May 10, 2016 at 1:42

There's also the issue of epiphenomena, that is events and behaviors that emerge from the collective action of numerous objects or continuous materials; surface waves on water are the classic example, but turbulence in general qualifies, and the mind also appears to fall in the category.

The upshot is that the term "exist" is heavily overloaded, referring to several different forms of existence drawn from multiple philosophical domains. When we say that rocks, numbers, physical laws, emotions, vibrations, corporations, and stories all "exist", we are exploiting that overloading. (There's a grammatical term for that sort of statement, but I forget it, and can't look it up just now.)

  • Dunno about the epiphenoma stuff, but "the term 'exist' is heavily overloaded" is exactly right.
    – user20153
    May 7, 2016 at 20:20
  • The word you are at a loss of is likely “autoholonymy.” Jun 27, 2022 at 8:26

If you have added the artificial restriction that only observable phenomena can exist (A premise that is not needed nor shown to be needed) then yes your forced the conclusion that non physical thing can not exist.

Be advised this blanket conclusion also arrives at the following.

  1. A universe or anything cannot exist with out a conscious mind.

  2. That existence is now restricted to each individual and does not reliably transfer between individuals. Yes there can be consensus on things such as "we all generally agree there is a sun since we saw it yesterday" your already relying on non physical memory to arrive at that consensus. Non physical being a restriction you discounted already.

Do you also arrive at the conclusion that the universe did not exist until conscious minds arrived on earth 13 billion years after the big bang? Although you can make the argument that conscious minds existed before humanity arrived on the scene I can say since that those minds are unobservable or rather has not been observed then it can not be proven to exist.

Although I understand your desire to discount unobservable things as being non existent (unreal) this leads us to even bigger existential pitfalls. :(

  • "If you have added the artificial restriction that only observable phenomena can exist" You misunderstand my starting premise. My premise is the following: Using logic alone, one can eventually prove anything, using suitably chosen axioms. Any statement can be shown to be true as long as we carefully choose are starting assumptions. The only way to put a limit on what is true and what is not, is if we limit our starting assumptions to empirical phenomena. Again: I am not saying only observable phenomena can exist, but that only observables phenomena can be used as starting assumptions. May 10, 2016 at 2:58

A rainbow exists. Although the sensation is the result of physical matter, colors do not exist outside of our perceptions. A hole exists. Although a hole is a concept describing the absence of surrounding matter, if the hole didn't exist we could not climb through a hole. A shadow exists. Etc.


Lets start by defining "physical" as perceptible by one or more human sense. Then, the answers to your 3 sub-questions are:
1 - What does it mean for something non-physical to exist? It means it is something that exists but it is not perceptible by humans.
2 - How can something exist without having a causal effect on the physical world? If it exists, it will have some physical effect, but we may not be able perceive the effect.
3 - Is there any way to prove the existence of something non-physical? Most of the time the answer would be no, but there are a couple of exceptions that I am aware of;
a) A man named Jesus Christ (and all his "works"), testified to the existence of an entity he called Father. He further testified that the Father and He, were ONE. He also said that the Holy Spirit would be sent to the apostles. Of these three "entities," one was perceptible to humans. We know of the existence of the other two, through the testimony of the one that was perceptible.
b) In Mexico city, there is a "painting" of Our Lady of Guadalupe, that was "painted by magic". But the "amazing thing" is not the whole painting, but the eyes. In the eyes, there is the image of the people She was looking at when the "painting" was made. This happened over 500 years ago! Long before humans had any notion of how the retina of our eyes acts like the film in a camera.
How the whole image and the images inside the eyes were "painted", has not been determined, as of late.

  • 1) A single electron is physical, yet not perceptible by human senses. 2) Then how do we now there was an effect? If it can't be perceived then there is no way of knowing it happened. 3)-a and 3)-b What you are describing does have a causal effect on the physical world, it is only the explanation that is lacking, not the effect May 10, 2016 at 22:22

Consider the alternative framing from idealism: "How can something that is not completely imaginary exist?"

If I perceive something, say a rock, I give it a reference in my head, and it becomes a thing to which I might refer. Nothing has been perceived that is not at least "whatever object that is at about 30 degrees left and 20 degrees up from the horizon." So what has or will have ever been perceived by anyone is all just a collection of references, linked to sensations -- it is an imaginary event.

But you know that you don't actually see that rock. You see some pattern of colors and feel some other set of sensations that just represent it. You imagine the rock given these hints, and that representation is just sensory content, linked to a reference -- it is an imaginary event.

Only your imaginary representation of the rock is ever perceived, and can ever be considered real. You have no business "deducing its existence logically" from anything. You have been tricked about this many times. In fact your deductions of existence require later correction so often that you should stop jumping to conclusions. You can really only reliably deduce that it only exists as an image.

We also say, or might potentially say, all kinds of things that are verbal content but do not refer to outside stuff "My favorite unicorn is indigo with salmon spots."

So the material is simply a subset of the imaginary -- and only in that sense can it be perceived, so only in that way does it actually exist.

It should be obvious, if you actually have the first question, that this answer is ultimately silly. But it is parallel to your own logic -- making that equally silly.

Language changes reality into something we can handle, and in the process, guarantees that we actually handle nothing else without layering it in language. There is no point in choosing only one of these domains to exist.

  • Silliness wasn't enough to dissuade Berkeley, and hasn't helped his detractors in any way. May 9, 2016 at 16:58
  • @AlexanderSKing This is where psychology is not irrelevant to argumentation: Berkeley would never ask the first question, either. So we are assuming this person is not an idealist and cares about sillyness. You cannot win the argument against idealism by inverting it. They win. But they reap the whirlwind.
    – user9166
    May 9, 2016 at 17:00

I'm no philosopher, and I cannot answer at the level of the other answers. But here is my short take.

The key problem with your reasoning, in my view, is: how do you define "physical"? For instance, one can take your reasoning and take it as a proof that God is physical.

We have a certain intuition about what "physical" means, but the deeper we understand our physics, the less clear this is. For starters, quantum mechanics shows us a "foggy", probabilistic reality, and it challenges our intuition about causality. And this is only in the well controlled environment of physical experiments.


Your question relates to the two domains of ontology and epistemology: Which kind of things exist? How do we know about the existence of things?

I propose to take Popper’s Three-Worlds ontology as a guide:

• World 1: physical objects and events

• World 2: mental objects and events

• World 3: objective knowledge and ideas created by the human mind.

See Chapter P2, of Popper, Karl; Eccles, John: The Self and its Brain. 1972 and the previous question Are there philosophies that call for things which are not mind nor matter?

Question 1: What does it mean for something non-physical to exist?

See the objects from World 3. E.g., ideas, are examples for existing non-physical objects. Ideas are special cases of the more abstract concept of information from World 3.

Question 2: How can something exist without having a causal effect on the physical world?

I do not know about any means to verify the existence of something which has no causal effect on physical objects.

Question 3: Is there any way to prove the existence of something non-physical?

Concerning information, verifying its causal effect is a method to prove its existence. Often, the causal effect of information is triggering a certain behaviour of the receiver. We even use a rough measure for information in our daily life: The number of Megabytes.


This question makes me think of Cartesian Philosophy and "intuition." It might seem odd that as humans we might make inferences rather than "pointing at the thing" but in fact as Descartes observed humans do this all the time. The English didn't really like the concept at all but it is quite decipherable..."God would never deceive us."

Imagine yourself a "soul-less being." According to Descartes that would be an impossibility. We breathe the air, we explore pleasures...we do so many things without thinking it's hard to imagine humans worried at all about "what is real" actually.

Descartes did explicitly separate the mind from the body...which strikes me as very dangerous...but perhaps he was trying to protect us from our "self"?

Of course that gets us into an entirely new realm of " non corporeal study" called psychology.

According to Science it is a proven fact that we have a mind actually and indeed have gone so far as to say we need doctors to "protect" it.

Yet if all we believe in is the corporeal then technically speaking not only do you not have a mind but you are incapable of reason or even experiencing things through for example "sensations."

Talk about limiting your scope of inquiry...

  • This does not appear to be an answer to the question. Or if it is one, it's extremely unclear.
    – virmaior
    May 19, 2016 at 5:01
  • Quite the opposite...it answers the question in the reverse by saying "the fact you have to think of something first before it can be true" means there is nothing that is "real" that a human cannot make "unreal" because to be human is to have to think about things first.There are physical forms and "sensory perception" but all of these matters only exist...according to Descartes...by "Imagineering" everything first. For example Microsoft co-founder Paul Allen has bequeathed a fortune to have the mind "mapped"... a very interesting concept indeed. Perhaps it is unclear because you have no soul? May 19, 2016 at 5:16
  • Perhaps it is unclear because you have no soul -- that definitely seems like the most likely explanation. But on this SE, we're trying to answer the questions people actually ask. How does this answer address how non-physical things can exist?
    – virmaior
    May 19, 2016 at 5:19
  • Do you believe in God? May 19, 2016 at 19:57

Gideon Rosen is one of the best writers in contemporary philosophy, and he has an excellent synopsis of the issue of abstract objects in the SEP.



To prove that an entity exists, one either has to observe it directly, or logically derive its existence from something observable.

We can't directly detect God, but we can logically conclude that God must exist from the fact of existence and its qualities. The physical realm can't have come from nothing or eternity. But something must be eternal, so there must be an eternal non-physical intelligent force that caused the physical realm.

And for free, we observe evil in the world, as in the actions of Adolf Hitler. The only rational way to account for those is the existence of a spiritual realm of good and evil. Theism accounts for this but atheism, which denies God, cannot.

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    Dec 20, 2023 at 9:16

The scientific method in based on repeated observations and experiment. The whole science is just a collectivist instrument of acquiring knowledge. Being an instrument, it has its own limitations. To satisfy the scientific method the result of a measurement should be communicated to other scientists using common language, and those scientists should agree on the results.

This means that something that cannot be communicated or explained in language is not physical.

For instance, a new sense of smell cannot be communicated via language, so it is not physical (but the chemical composition of the substance that produces the smell is physical).

Another case is when the experiment cannot be observed by multiple observers or reproduced. For instance, if something leads to disappearance of the observer (and his instrument) or split of the observers into parallel universes, and the like.

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