What is the fundamental difference between something that exists and something that doesn't exist?

What exactly are we claiming when we say that something "exists"?

Is there a distinctive property that all existing things possess that all non-existing things don't possess?

The concept of existing is certainly a human invention but I can't identify what we are trying to point out with this concept.

Any ideas?

  • 1
    A very tough question, and I’m not sure if an answer can be given. Something you might want to look up is W. V. Quine (1948), ‘On What There Is’, if you haven’t already.
    – Quae
    Aug 22, 2022 at 18:42
  • 1
    "Affect" is kind of vague, one can get "affected" even by unicorns, I suppose. If you want to express it more specifically you'll have to distinguish different kinds of existence, any description that covers both rocks and numbers will be unavoidably vague, see What is existence and what kinds of it can be distinguished?
    – Conifold
    Aug 23, 2022 at 0:05
  • 2
    Since you never see and feel the scalar potential energy (PE) then how you're sure it exists? Per analytic philosophy it only makes sense to claim existence or not within a certain language framework, thus PE exists in the framework of (public) physics. As Wesley C. Salmon claimed to exist is to be self-identical in free logic. Thus insofar as in the common language domain your concerned entity is a non-empty constant it exits, and thus unicorn doesn't exist since you cannot find a non-empty constant in the common language which is identical to unicorn... Aug 23, 2022 at 1:04
  • 1
    It seems the question is exactly that, to clarify the concept of existence.
    – Florian F
    Aug 24, 2022 at 7:13
  • 1
    @JD Gosh, and they haven't managed it yet? People complain about the government, but that's just peanuts to Philosophy.
    – Scott Rowe
    Aug 25, 2022 at 0:59

7 Answers 7


Following Quine, a fairly popular approach to the question of what exists is to say that those things exist that are indispensable to our best scientific theories. On this view, electrons exist because our best account of fundamental physics includes them. Water exists, but phlogiston does not. Composite objects such as tables and kangaroos exist, because they are made up of fundamental parts, though it is notoriously difficult to say what constitutes the identity conditions of a composite object. There might be tricky cases where it is disputed as to whether a things exists, but on this view, the ultimate arbiter is science.

Abstract objects such as numbers can be said to have a kind of existence, according to Quine, since we need them to do science. Although that claim has been disputed by Hartry Field among others.

Quine's approach to ontology is very minimalistic, and his account still leaves many questions unanswered. Do universals exist, independently of particular instances of them? Do minds exist? In what way might we say that fictional objects exist? Is there a realm of non-existent objects or potential objects that subsist or have some second tier of being? Philosophers disagree about how to handle examples such as these.

Also, scientific theories are underdetermined by data, so it might be possible for two equally successful scientific theories to entail the existence of different kinds of objects.

As to your last question about what we are trying to do by talking about existence, the question of what exists seems to be very fundamental. We tend to think about our experiences, and attempt to make sense of them, by supposing that there are things, properties of things, and relations between things. Maybe it would be possible to create an account of our experiences in other terms, such as ideas or processes, but for most people it seems natural to accept that there are things, and these things have properties and they participate in relations. It is not an accident that our best logics work this way: we quantify over things, and use predicates to express their properties and relations. Once this basic framework is in place, the whole of our knowledge rests on the question of what things does our universe consist of.

  • 2
    Is this really an ontological theory? It strikes me as an epistemological theory: not "what does it mean for something to exist?", but "how do we determine whether something exists?". Aug 23, 2022 at 11:22
  • I think for Quine at least, it is ontological. In his papers, "On What There Is" and "Ontological Relativity" he advances his characteristic position that "to be is to be the value of a variable". In other words, what defines an existing thing is that it is what we quantify over when we express our best scientific theories in first order logic.
    – Bumble
    Aug 23, 2022 at 13:25
  • 1
    Quine is an interesting starting point, but since he wrote, both reductionism and scientism have been rejected by the relevant expert communities. For scientific reduction it is philosophers of science, see section 5 of SEP on Scientific Reduction. For scientism (science is sole source of knowledge about world) I doubt one can find more than a handful of advocates today. Apply Quine to our current pluralist knowledge environment and one runs straight into a plurality of different “reals”.
    – Dcleve
    Aug 23, 2022 at 13:58
  • Bumble "those things exist that are indispensable to our best scientific theories" This is clearly not what we think of as existence. You might just as well say that existence doesn't exist and limit yourself to talking about the necessary grounds for doing science. Aug 23, 2022 at 16:32
  • 1
    @PhilipKlöcking Kant already warned the potential vanity of epistemology-only synthetic a priori philosophy in his rare straightforward passage in his Critique of Practical Reason: Nothing worse could happen to these labours than that anyone should make the unexpected discovery that there neither is, nor can be, any a priori knowledge at all. But there is no danger of this. This would be the same thing as if one sought to prove by reason that there is no reason... Aug 25, 2022 at 2:27

There are several accounts of existence. The two most popular are probably the classical notion and the Frege/Russel notion.

The classical notion is that existence is something that some objects have and others don't. Horses have this something and unicorns don't. Some philosophers have noted that this something can't be a property because an object has to exist before it can have any properties; that is, a thing has to have existence before it has properties, or in order to have properties. There have been hundreds of thousands of words written about what this something is and what sorts of things have this something.

The Frege/Russel notion of existence is that existence isn't something that individuals have; it is just a higher-order predicate. According to this notion, it is not, for example, cats that have existence, but rather that the set of cats is not empty or that the concept of cats is instantiated. It is simply a logical notion.


What condition(s) must be met to claim that something exists?

The conditions for making any claim whatsoever are that you be able to make that claim. This is probably not the answer you are looking for but this is the question you asked.

Maybe we could rephrase your question?

What condition(s) must be met for something to exist?

There cannot be any condition on something that doesn't exist because it doesn't exist.

There cannot be any condition on something that exists for it to exist because it already exists.

Conditions cannot be on reality since reality already exists and exists as it is, which includes the existence of any particular thing.

Maybe we could ask a somewhat different question?

What condition(s) must be met for us to know that something exists?

This is essentially the question addressed by other answers.

We can begin by observing, à la Descartes, that being conscious involves knowing that you are conscious and so involves knowing that you exist as a conscious thing. Are there any conditions? Not any that anyone would know except obviously that you need to be conscious. We may want to say that you need to perceive the world around you but no. It is possible to be conscious without perceiving the world around you, and this seems enough to know that you exist as a conscious thing.

However, this does not solve the problem of the conditions for us to know that other things than our own consciousness exist. What are the conditions for us to know that the Moon exists? It doesn't seem that we know the answer to that because it doesn't seem that we even know that the Moon exists to begin with.

We certainly know the mental image that we naïvely take to be the Moon but we know it as part of our own consciousness, so we are going back to the initial resolution of the conditions for knowing that we are conscious. And that we should know some content of our own mind does not imply that we should also know that whatever it may seem to represent or signal also exists.

So, it seems that the condition for us to know that something exists is that it be somehow part of our own consciousness, and then we only know that it exists at the moment that we are conscious of it. If something is in our own consciousness, then we know that it exists (as such).

However, our concept of existence goes beyond existence as mental object. We believe that the Moon exists somehow outside our own mind. Perhaps thanks to philosophy and science, we also realise that the Moon we are conscious of is not the real Moon, if this one exists at all. We also realise that the real Moon is unlikely to be much like the Moon we are conscious of.

The most plausible conclusion is that we cannot possibly know that the real Moon exists. We can believe that it does, and we do, we may even be "dead certain", but, presumably, we cannot know that it really exists.

That being said, it doesn't seem to matter in any way.

It doesn't matter because we don't need to know whether the Moon really exists. What we need is essentially to survive, prosper and reproduce. Our beliefs seem on the whole good enough to help us achieve that. Humanity seems to have survived, prospered and reproduced for more than 300,000 years. Life has existed for several billion years. Animal species which have a brain and rely on their beliefs about their environment may have existed for several hundred million years. So, there is no doubt that the sort of beliefs we have work. No only that, but we broadly understand how they can be so operationally effective.

So, a better question could be:

What are the conditions for our beliefs to be operationally effective in helping us survive, prosper and reproduce in the real world?

This seems a much more tractable question and one which is essentially the subject of various sciences, including the cognitives sciences, formal logic, evolutionary biology etc., and the actual answer to this new question will emerge with every new result coming from these sciences.

However, a basic answer is that the beliefs we develop as a direct result of our perception of our environment provide an effective basis for our survival, our prosperity and our reproduction. Technological and scientific progress, and the operational effectiveness of our technology and our science also seem to prove that we need to stick to the scientific method for developing our beliefs about the real world. The scientific method is really just our ordinary, native rationality applied in a systematic way. Science is just rationality plus organisation, cooperation, professionalism, use of our technology etc., and, crucially, memory of our science beyond the life span of individual humans through successive generations. And rationality is essentially facts plus logic.

These are the conditions.

These are the conditions not to know that things exist, but certainly to be able to trust our belief that the Moon exists to the point that we can send a man to land on it and come back alive with a handful of Moon dust.

  • Great answer! Just a curiosity/nitpick - you said: "[...] various sciences, including the cognitives sciences, formal logic [...]" - Is formal logic a science? I believe it is not (as far as I concluded from an introduction to epistemology course in the past).
    – Pedro A
    Aug 24, 2022 at 3:18
  • What is the difference between "the Moon exists" and "the Moon really exists"?
    – Florian F
    Aug 24, 2022 at 7:39
  • Also when accepting this answer the question becomes not what exists, but what is the source of this thing which exists(in my mind). It can be external and thus have some form of objective reality or it can be internal, sensory noise, optical illusion etc. Because nobody can claim that what somebody experiences and thus his experience "exists" doesn't exist. But they for sure can claim it does not correspond to an external entity.
    – Hakaishin
    Aug 24, 2022 at 8:22
  • @PedroA "Is formal logic a science? I believe it is not" If think of formal logic as the science of the logic underpinning human reasoning, the empirical science of logic. This is certainly how it is thought of by many academics working on the logic of human reasoning, essentially in cognitive sciences, in linguistic, on argumentation theory etc. This is to be distinguished from mathematical logic, which is mathematics, not formal logic, and so not an empirical science. Aug 24, 2022 at 10:31
  • @FlorianF "What is the difference between "the Moon exists" and "the Moon really exists"?" the word "really"? Statements don't have meaning. Rather, we use them to mean whatever idea we want to convey to others. These two statements may mean different things to different people. The word "really" will be used mostly to emphasise the distinction between things existing in the so-called "real world" and things existing only in human minds. The Unicorn may be said to exist as an imaginary being, and not really existing, as the English Queen may be said to really exist. Aug 24, 2022 at 10:41

Perhaps the best definition of existence is that attributed to George Berkeley: esse est percipi (existence is perception).

If you perceive a hammer with your smashed finger, it exists. If you feel sadness, it exists. If you perceive an opportunity, it exists. If you don't perceive an irregularity on a surface, it doesn't exist... for you. If you don't perceive dark matter, either with your instruments, your reason, equations, your senses, then, it does not exist.

Notice there is a difference between empirical existence (what the senses perceive: unicorns vs. horses) and plain existence (what any form of intuition allows perceiving). Commonly, science refers to the former, but in any other case, it's about plain existence (e.g. crisis, order, poverty, beauty, respect, danger, numbers, etc. can't be perceived by the senses, but do exist for most). For example, quarks can't be perceived by the senses, but they do exist because science tells so.

Notice existence is subjective: UFOs, beauty, danger or God are perceived by some, not by all. That is, some believe they exist, some don't.

  • 1
    Perception is a very suspect criteria for existence. We only perceive the world locally and with a narrow range of sensors. Magnetic fields are clearly “real” but imperceptible to humans. And I can imagine a stop sign, it’s coloration, surface feel, hardness, vibration when I tap it, grime an corrosion around its bolts, etc. I can do this because perceptions are manufactured. Neurology has characterized the way we build up object models unconsciously from our very different raw detector data. Imagination is able to feed into the output end of this buildup process.
    – Dcleve
    Aug 23, 2022 at 15:07
  • @RodolfoAP 1. "existence is perception" This is clearly not what we think of as existence. 2. "it doesn't exist... for you." This is clearly not what we think of as existence. You might just as well say that existence doesn't exist and limit yourself to talk about perception. Aug 23, 2022 at 16:27

My understanding has always been that something exists if it has at least one measurable property. Hence a brick has size and mass, a photon has wavelength and energy, and my lifetime may be measured in years. That’s not a great definition since it depends on what tools you have to make measurements but it’s a start. It does allow for the intriguing possibility of having multiple frames of reference that don’t overlap at all.

  • 1
    But a round square which doesn't exist also has properties namely the properties of having a round and square shape.
    – WokeBloke
    Aug 23, 2022 at 11:52
  • Yes, set theory allows us to create sets that are paradoxical and nonsensical but that doesn’t reflect back on reality. I would submit that non-existence is not a property although that’s a fascinating concept
    – Frog
    Aug 24, 2022 at 10:24
  • @WokeBloke Fascinating... can you name a single measurable property that a round square has?
    – philosodad
    Aug 24, 2022 at 19:10
  • @philosodad How about a round square with a radius of 20 meters and a side length of 10 meters?
    – WokeBloke
    Aug 24, 2022 at 21:12
  • @WokeBloke Can you define "radius", here? As far as I know, in geometry, a 'radius' is a line segment from the center of a circle or sphere to its edge, and you aren't describing a circle.
    – philosodad
    Aug 25, 2022 at 16:41

I think, the question is not well posed: The word "exists" has several different meanings.

  1. Something exists if it's part of our universe. As such, horses exist, but unicorns don't. This meaning of the word is relatively straight forward. However,

  2. ideas and abstract things are said to exist as well. Numbers exist. Several classes of numbers exist (natural, integer, rational, irrational, transcendent, complex, etc. pp.). Other mathematical constructs exist. The ideas of wizards, vampires, unicorns, homeopathy exist in the minds of humans.

  3. Things can exist in imaginary worlds. There is a philosoper's stone in Harry Potter. There are also magic wands. There is also a ring world somewhere far away where people walk on the inside of a mind boggling large, tremendously fast rotating ring. Ask Larry Niven about it.

As such, heated arguments about whether something exists or not are pretty mute. If you want to use the word "exists" in a negative sense, it's better to say in which context it does exist. I.e. a philosoper's stone exists in Harry Potter's world, but not in ours. Likewise, vampires exist in various forms in many different books, but luckily, they won't come to suck your blood. And mathematicians create abstract concepts simply by definition, so you cannot claim that imaginary numbers don't exist. They exist in mathematical and physical formulas, and they are dead useful. I've never talked to an imaginary number, though.

  • What about the universe itself? It isn't a part of our universe but it still exists right?
    – WokeBloke
    Aug 24, 2022 at 6:51
  • @WokeBloke Exactly. There are so many ways in which we can say that something does (not) exist that it's best to define how you intend to use the word. Tangent: I would say that this question has more to do with linguistics than with philosophy. You are basically asking for the precise definition of the word "exists" while there is none that everybody could agree upon. And there are many things that exist in one sense of the word, while they are purely imaginary in another sense of the word. But if you say "I'm going to use this definition [...]" and stick to it, you are good. Aug 24, 2022 at 10:04
  • Perhaps if we can't agree on the use of a word, we shouldn't use it? If 'exist' is so problematic, just don't bother with it.
    – Scott Rowe
    Aug 24, 2022 at 16:50
  • @ScottRowe I don't think so. Every word in a natural language is a bit fuzzy. And if you replace your use of "exists" with some other word or phrase, you are probably not any more precise and/or unnecessarily verbose. The correct fix is to say which definition you are going to use, so that you and your readers/listeners are on the same page. This allows you to be as precise as you want, it's applicable to any word that needs more precision, and is the standard way to handle this problem in the sciences. Aug 24, 2022 at 20:12
  • @WokeBloke "What about the universe itself? It isn't a part of our universe but it still exists right?" It can be argued that the universe itself doesn't exist - except as a word that was invented to include everything that does exist.
    – D. Halsey
    Aug 25, 2022 at 0:43

I would say anything that can be perceived from six senses " touch, sight, hearing, smell, taste or thought " can be claimed of existing.

For example :

I see a bat in front of me so I claim that the bat exists .

I smell an odor, I claim their are tiny particles of that odor in the surrounding .

I taste something so I claim there exists something in my mouth.

I hear people talking across the room so I claim there are people across the room.

I touch something soft, round and jiggly so I claim its a Balloon.

A 'x' religion person thinks that the 'x' religion god is real so he tries to prove the gods existence through thought/argument for example who created the tree, oceans etc.

  • But what about things that exist that we don't perceive? What is the commonality between those things and the things we do perceive?
    – WokeBloke
    Aug 22, 2022 at 17:40
  • 3
    Can you account for molecules, atoms, and quarks? Do they exist? How about quantum fields?
    – user4894
    Aug 22, 2022 at 23:27
  • 1
    "…a human perceives from his five senses…" Are you aware that humans have more than five senses?
    – Sandejo
    Aug 23, 2022 at 0:29
  • 2
    Circles, numbers, holes or love do exist, but can't be perceived by the senses.
    – RodolfoAP
    Aug 23, 2022 at 4:44
  • This is a basic or say general example, your questions are very specific I would say, which can not be put into simpler words, though I shall add the sixth sense of thought/thinking.
    – AbduRahman
    Aug 23, 2022 at 8:26

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