What are the criteria for existence, i.e. the answer to "what exists and what doesn't exist?" in modern schools of philosophy?

My trial: Something exists if and only if it can affect our senses, either directly or indirectly. For example, magnetic fields or electrons exist, because they affect our measuring devices, which we can see.

Update 1: There are at least two challenges for this criterion:

  1. Does a nation exist? What about a city, a corporation or a family?

  2. Do dragons exist? Their existence is not logically impossible. Additionally, if they exist, they can affect our senses.

Update 2: Another approach:

We need to accept the existence of at least one object as a reference (let's say our hands), which is followed by two rules:

A: Anything observed to have interactions with existent objects is also existent.

B: Anything whose existence is consistent with our knowledge probably exists.

I think the circularity of this definition is resolvable, but I admit it's vague.


15 Answers 15


It is important not to conflate metaphysics with epistemology. Your question asks for the "criteria of existence" and seeks a set of criteria for determining when something exists. However, you then posit a set of operational criteria not for existence itself, but for obtaining knowledge of an existent. That is an epistemological ---not a metaphysical--- issue. As a metaphysical issue, there can be no "criteria" for existence since it is not up to us to decide what rules reality must conform to in order to exist. Existence merely exists, and is whatever it is --- it obeys whatever principles are in its nature to obey and it is for us to discover its nature. (One might cleverly argue that the fundamental nature of existence forms the "criteria" of existence, but that would be putting the matter backwards.)

In regard to what particular schools of philosophy say about the nature of existence, that is virtually the entire subject of metaphysics (or at least the main starting issue in this field). There are many ideas that have been put forward by different philosophers (and schools of philosophy) throughout history, and it would be difficult to summarise this discussion in a short post, since that would mean recounting the history of the subject of metaphysics. While I won't attempt to do that, perhaps I can nonetheless give you a taste of it by considering a couple of common issues and positions.

Is existence a property? To start with, there has been historical debate within metaphysics as to whether "existence" is a property of a thing or a precondition of a thing. Plato believed that the "forms" for various things existed apart from actual objects. Similarly, some of the medieval philosophers (e.g., Aquinas) held that there was a distinction between existence of a thing and its "essence", and they held that a thing could have essence but not existence. Hence, they regarded "existence" as a property of a thing (e.g., a horse has this property but a dragon does not). Other philosophers (e.g., Hume and Kant) regarded this as an illusory distinction, and regarded existence as not being a property, but a precondition for having properties.

The noumenal and phenomenal worlds: Once you transition from metaphysics to epistemology, you will encounter Kant's famous arguments about the relation of existence to perception. Kant believed that existence was split ---epistemologically speaking--- between what we perceive with the senses (the phenoumenal world) and things as they are "in-themselves" (the noumenal world, unmediated by the senses). He regarded the noumenal world as the "real world", and denied that we can have knowledge of this through the senses. ("I have therefore found it necessary to deny knowledge, in order to make room for faith.")

Is existence all in the mind (idealism)? Berkeley made a similar argument to Kant, but used it as the basis for the idealist claim that existence is merely a mental construct. Berkeley argued that the mind "cannot conceive of bodies existing unthought of", and therefore its only apprehension was of a special kind of existent that exists only in the mind. (The philosopher David Stove famously called this form of argument "the worst argument in the world".)

As you can see, there are some arguments about existence which use the duality of what can and can't be perceived to ground a theory of the limits of human knowledge of reality (e.g., Kant's phenomenal world). Arguably the idealist position attempts to elevate this to a claim about the criteria for existence itself, but even here it is usually framed as existence "to the mind".

  • I think I might have some positivistic attitude toward metaphysics. The type of knowledge of existence that I'm concerned about is binary; exist or not. Isn't it self-contradictory to believe (know) there exist things that their existence is not knowable?
    – asmani
    Commented May 7, 2018 at 11:24
  • 1
    I wouldn't say it is contradictory to believe that things could exist that you cannot know about. To hold otherwise is to impose an epistemic requirement for existence, and again, this is a matter of dictating to reality what it is allowed to be. Having said that, epistemologically, it is reasonable for us to impose rules as to admissability of concepts into discussion. The positivists argued that one should restrict attention to "positive" knowledge of phenomena (i.e., they accepted Kant's dichotomy and then said, okay, then discussion of the noumenal world is inadmissible).
    – Ben
    Commented May 7, 2018 at 12:07

I would prefer a definition of existence which is independent from human senses as test device. Because the world exists since billions of years when no humans lived.

Therefore I propose a generalization of your criterion:

An object "exists in the physical world" means: It "interacts" with other objects.

This definition is expanded in the book "David Deutsch: The Fabric of Reality, Penguin 1997" in the chapter Criteria for Reality. Deutsch writes: "It [a certain example] also illustrates the criterion for reality that is used in science, namely, if something can kick back, it exists."

But on a deeper level, giving a precise definition of existence is a difficult task. Hence sometimes one takes existence as undefined basic term in the sense of being real, in contrast to being only possible. And the question of the existence of general concepts, e.g. of ideas or mathematical objects, is an issue of perennial philosophical discussion.

  • 2
    What is the definition of an object? Is it different from existent?
    – asmani
    Commented Apr 27, 2018 at 19:50
  • @Asmani What about „A hypothetical entity is real, i.e. an object, if if can be considered the source of the affection of an other object“?
    – Jo Wehler
    Commented Apr 27, 2018 at 20:44
  • Actually, possibility in mathematical sense is the same as existence. Everything you can imagine exists in mathematical sense. Taking physics it might be not very different if we assume all our thoughts are contained in some imaginary worlds.
    – rus9384
    Commented Apr 27, 2018 at 23:19
  • @rus9384: I believe we're using the diamond operator for "possibly," rather than the more mundane existential quantifier.
    – Kevin
    Commented May 3, 2018 at 6:11


1 A number exists if it belongs to the conceptual body of number theory in which it can be referred to in true propositions. Then does Zeus exist if Zeus belongs to the conceptual body of Greek myths in which Zeus can be referred to in true propositions such as that Zeus was the son of Kronos ? The same kind of question can be asked about dragons and unicorns, which belong to various conceptual bodies of mythology. And the answer is the sane, 'No,' because number theory is true in some sense (from Platonic to Pragmatic) in which the conceptual body of Greek myths and the conceptual body of myths about dragons and unicorns is not. Whatever existence a number has, it is a conceptual existence.


2 An object exists physically - has physical existence - if it occupies space/ time. Typically physical objects interact, but what of the whole of physical reality ? If that exists there is nothing for it to interact with.


3 There is also the question of whether and how abstract objects such as universals exist. On one view redness, say, or circularity, is an abstract object or entity which, though it has no spatial location, is instantiated in past, present and future red or circular things. The existence of universals conceived in this way is a controversial matter, and so there is no straightforward, uncontentious way of specifying their mode of existence.


The Questioner has instanced in comments the case of Russell's Teapot* and adds, 'I think we should somehow include evidence in our definition' (my emphasis). Evidence for a hypothesis or theory exists, as evidence, if it supports or undermines the hypothesis or theory; that is, if its truth or presence makes the hypothesis or theory more or less likely. I could formalise but won't. For it seems to me that the request for criteria for the existence of evidence opens immediately into a request for criteria for the existence of the mental. Evidence is only evidence in relation to an inquiry or an interest, and as such presupposes the existence of mind.

Now, in the current state of the philosophy of mind, mind is an essentially contested concept. If the mind/ brain identity theory is correct - i.e. if a particular version of physicalism is true - then the mind is a physical object and the criteria for its existence can be followed up under 2 above.

If the mind is distinct in some way from the body (from the brain and the central nervous system) then until we decide in what way it is distinct (is it a Cartesian substance, an epiphenomenon, an emergent property ?) there is no hope of fixing the criteria for its existence. Perhaps mental phenomena are distinguished by incorrigibly private access or by their intentionality, their 'aboutnesss' (a thought is about something, real or imaginary). None of these matters can be settled until there is (enough) agreement about what the mind is and whether we actually have minds distinct from bodies.

[*Bertrand Russell, 'Is There a God?' : Many orthodox people speak as though it were the business of sceptics to disprove received dogmas rather than of dogmatists to prove them. This is, of course, a mistake. If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time. (https://en.wikipedia.org/wiki/Russell%27s_teapot.)


Amie L. Thomasson, 'Existence Questions', Philosophical Studies: An International Journal for Philosophy in the Analytic Tradition, Vol. 141, No. 1 (Oct., 2008), pp. 63-78.

Penelope Maddy, 'Mathematical Existence', The Bulletin of Symbolic Logic, Vol. 11, No. 3 (Sep., 2005), pp. 351-376.

Chris Mortensen, 'Explaining Existence', Canadian Journal of Philosophy, Vol. 16, No. 4 (Dec., 1986), pp. 713-722

R. I. Sikora, 'Rorty's New Mark of the Mental', Analysis, Vol. 35, No. 6 (Jun., 1975), pp. 192-194.

Stephen Mumford, 'Intentionality and the Physical: A New Theory of Disposition Ascription', The Philosophical Quarterly (1950-), Vol. 49, No. 195 (Apr., 1999), pp. 215-225.

  • Thanks. Does the Russell's teapot exist? I think we should somehow include evidence in our definition.
    – asmani
    Commented Apr 29, 2018 at 14:22
  • Hi : why shouldn't evidence consist of objects having physical or possibly conceptual existence ? Evidence doesn't seem to me to be something extra to such objects. But you may have other thoughts in mind ? Indeed, if thoughts exist, we need criteria for their existence.
    – Geoffrey Thomas
    Commented Apr 29, 2018 at 15:03
  • I mean the Russell's teapot does occupy space/time as a property, but we need evidence to admit it exists.
    – asmani
    Commented Apr 29, 2018 at 15:31
  • And I meant that that evidence would consist of physical or conceptual objects. Nothing extra to what I had mentioned. Do you mean that we would need perceptual evidence involving minds and thoughts, perceptions and sensations ? If so, that was implicit in my question about thoughts.
    – Geoffrey Thomas
    Commented Apr 29, 2018 at 17:45
  • Thanks for the update. I'm not sure if I understand your explanation. I'm not saying that we should add a category under evidence. I'm concerned about how the knowledge about existence of physical objects is gained. How to know if the Russell's teapot exist? If your definition is "An object exists physically if it occupies space/time as a property" then we conclude the Russell's teapot exists. If you mean it is the case that it occupies space/time, then it doesn't. However, to know if it is the case or not, we need to appeal to evidence.
    – asmani
    Commented Apr 30, 2018 at 9:52

Yes, "affects our senses" would indeed be the effective operational definition of "existence". But don't forget (a la JoWehler) that we can interpose experimental apparatus (sometimes very complex, like the LHC) between the phenomenon that putatively "exists" and our senses. For example, we can't directly see/sense atoms (i.e., directly = without intervening apparatus), but we can directly see image evidence (electron or atomic force microscope images, etc) of atoms. And in that indirect sense, atoms directly affect our senses.

So you have to look at the "equivalence class" ( e.g., https://en.wikipedia.org/wiki/Equivalence_class ) of the "affects" relationship. That is, if "A affects B" and "B affects C", then by transitivity, "A affects C". And by extension, with "-->" denoting "affects", A-->B-->C-->...-->Z. So if A denotes some "element of existence", and Z denotes our direct sense experience, then any number of intervening experimental apparatus still preserves the "existence" property.

And that idea permits us to partition existence into (at least) two equivalence classes: things that "affect our senses" (directly or indirectly), and things that don't. But if "things that don't" aren't the empty set, how are you going to establish their existence in the first place??? By definition: i.e., you can't do it observationally, so you're left with logical (or sometimes not-so-logical) argument. And those kinds of (logical) arguments would ultimately boil down to definition, e.g., do "numbers" exist (and, if so, integers, rationals, reals, etc)?

And, to my mind at least, "existence" has to be, in one way or another, more than mere definition. And therefore, "affects our senses" (directly or indirectly) must be the ultimately correct criterion.

  • 1
    I tend to assume that the criterion is applied to descriptions. We have a description of dragons, which lies in the set of non existent. However... I will add an update to my question.
    – asmani
    Commented Apr 28, 2018 at 10:39
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    @Asmani Yes, descriptions are crucial. But not, as you might be thinking, of the thing (dragons) that putatively "exists" (or not). Rather, descriptions of the construction and operation (and set of possible outcomes) of the apparatus used to detect that "thing". See, e.g., researchgate.net/publication/… which is the earliest paper I'm aware of about this (Schwinger has some similar stuff in the 1950's). The same basic idea has more recently been re-christened "test spaces", e.g., Alexander Wilce has published lots under this rubric.
    – user19423
    Commented Apr 28, 2018 at 10:52
  • 1
    @JimGarrison You talkin' to me???!!! :) Then I'd say "no". You might hold "1 apple" in your hand, or "1 book", etc. But you ever try holding just plain old "1" in your hand? "1", per se, isn't detectable by any apparatus (including your senses). Detectable things that (in my opinion) "exist" have units/dimensions, e.g., grams, centimeters, seconds, coulombs (or apples, books), etc. But "1" is dimensionless. However, if you've got some other definition for "exists", then that's your business. Whether or not you can make a successful business out of that is another question altogether.
    – user19423
    Commented Apr 28, 2018 at 21:47
  • 1
    What is the utility of a definition of "existence" that fails to include abstract concepts? Certainly they exist in the sense that many people can have a shared experience of them. Or, consider the software that underpins the entire world... the media on which it resides are tangible artifacts, but the software itself is not something you can perceive using only your senses. Does it not "exist"? Commented Apr 29, 2018 at 0:25
  • 1
    @JimGarrison The >>thought<< of a number (or abstract concept) "exists" (again, in my opinion, by my definition (oops, n.b., not my original idea, just the one I'm advocating here)) because the thought corresponds to a physical brain state that can be observed/detected by an fmri, or more simply, just by asking me what I'm thinking and hearing what I say in response. But the number or concept doesn't "exist" absent anybody thinking about it. Whereas atoms, molecules, etc, do independently exist. (Similarly for your software ~ media.)
    – user19423
    Commented Apr 29, 2018 at 1:13

I think there is a fundamental problem in your question: Namely the implicit assumption that there is only one form of existence, that is, that there's one meaning of "exists" which applies to everything of we say it exists. Which I think is wrong.

For example, if we say "the sun exists" we mean something very different from what we mean if we say "a solution to this equation exists", which is again something very different to what we mean if we say "love exists".

Therefore I think a single criterion of existence cannot exist (in the logical sense). Instead, existence is a spectrum, where on one end we have physical existence like the existence of the sun or of atoms (although even that can be less straightforward that one might think at first; e.g. in a quantum superposition between an not yet decayed neutron and one that decayed into a proton and an electron, does the electron exist or not?), and on the other end we have logical existence (like the existence of prime numbers, or the existence of criteria of existence). OK, maybe the spectrum is actually multi-dimensional (so you cannot actually speak about "ends", only about extremes).

However I think there's one necessary criterion of existence that all forms of existence must pass: Can you lie about it? I think that if you cannot lie about something, it's clear that it cannot possibly exist.


I realize that Seamus has already mentioned the classic criterion proposed by Quine, but I think Seamus may have slightly misrepresented it; so I thought I might add a few more details. It is true that Quine famously states "to be is to be the value of a variable" in is seminal paper "On What There Is." But I think it is slightly inaccurate to say that Quine thought that whatever exists is whatever is referenced/quantified in our best scientific theories, that the set of existing things just is the set of all that is quantified in our scientific theories. Here's what Quine says:

Now how are we to adjudicate among rival ontologies? Certainly the answer is not provided by the semantical formula "To be is to be the value of a variable"; this formula serves rather, conversely, in testing the conformity of a given remark or doctrine to a prior ontological standard. We look to bound variables in connection with ontology not in order to know what there is, but in order to know what a given remark or doctrine, ours or someone else's, says there is; and this much is quite properly a problem involving language. But what there is is another question.

In other words, Quine is saying that the criterion "to be is to be the value of a variable" is more or less a way of determining what are language commits us to ontologically, specifically, the referential/singular terms and quantified variables employed in language. So there may be existing things that may go 'undetected' by this criterion precisely because they haven't been referenced in our best scientific theories. For Quine, if there was no way of paraphrasing a instance of reference or quantification in a way that avoids that reference or quantification, then it has to exist. In fact, this is springboard for the Quine-Putnam Indispensability argument for Mathematical Platonism. Roughly, the argument is that, since our best physical theories contain existentially quantified propositions whose domain of quantification consists only of abstract objects, these abstract objects must exist.

I think Seamus is correct when he says that this is the view accepted by most philosophers, but that isn't to say it has gone entirely uncontested. As with any philosophical thesis, the Quinean criterion for ontological commitment has been challenged. E.g., William Lane Craig in his relatively new book "God and Abstract Objects" launches an attack on the Indispensability argument precisely by disputing the Quinean criterion for ontological commitment. I think William Alston, Jodi Azzouni, and Penelope Maddy are others who take issue with it too.

Now, let me take a stab at (partially) answering your question. Your criterion seems to precludes the existence of abstract objects. In the comments on Dheeraj Verma, user193319 brings up this very point. However, Dheeraj Verma seems to misunderstand what abstract objects are. He states that abstract objects exist through the mind. First of all, it isn't entirely clear what "existing through" consists in; this seems to mean they have existence by virtue of us thinking about them, which is a very mistaken view of abstract objects. Abstracts objects, if they exist, do so necessarily and independently of whether they are conceived of by contingent human minds. Moreover, we don't know abstract objects exist because we interact with them by thinking about them; this is impossible because abstract objects don't stand in causal relations. Rather, we know they exist because we posit their existence because they fulfill some role (e.g., the role of explaining why mathematical truths are true and why they are necessarily true). Finally, note that the Indispensability argument, if successful, shows that your 'sensory' criterion doesn't work, since sensory affection is ultimately what justifies science, yet this in turn shows that abstract objects exist, which are causally inert and therefore cannot interact with our senses.

First EDIT:

In this edit I will try to answer the question Asmani posed in his comment. It gets tricky, however, since we begin to skirt issues in philosophy of mathematics and physics. Consider Newton's (differential) equation F=ma. You might imagine the following said in a physics course: "Such-and-such a particle is following this trajectory in space, and there exists a solution to Newton's equation describing its trajectory." These seem like to true statements. We might symbolize the last statement using logic in the following way: letting S be the set of all solutions and "s is a solution describing the particle's trajectory", the last sentence reads Now, the basic idea behind this ontological principle is that the existential quantifier ranges over S, and S only consists of abstract object (namely, solutions to the differential equation). Since the statement is true, there has to be a solution (i.e., value of the bound variable) which makes it true and therefore there exists an abstract object.

I confess this is a somewhat artificial/contrived example, but I think it captures the gist of what "to be is to be the value of a variable" means and how this principle of ontological commitment is to be used; moreover, it shows how the Indispensability argument works. Indeed, I feel compelled to add that Quine himself would deny that this is a genuine example. Though Quine was a staunch Naturalist, he saw the need of abstract objects because of the success and truth of science; but he didn't want an 'extravagant' or 'inflated' ontology. Because he was able to reduce mathematics to set theory, he could see such things differential equations and their solutions as complicated logical constructions of set theory.

I'm afraid I can't go into all details on the process whereby higher mathematics are reducible to set theory. But, e.g., the natural numbers can be reduced to sets in the following way: 0 is taken to be identical to , the empty set; 1 identical to ; 2 identical to ; etc. From the naturals you construct the rest of the integer, rationals from the integers, etc. So in Quine's ontology, there aren't numbers but pure sets, since numbers are in a sense logical constructions.

Sorry for the extraordinarily long post--I have no one else to discuss philosophy with, so sometimes I go overboard; and sorry if this seems like a complicated example, but when I first read your comment, I was thinking you wanted an example in the context of the Indispensability argument.

Second Edit:

In answer to Asmani's request about a physical object. Well, it's somewhat tricky. I think it depends upon context to some degree. Typically, using the word "this" connotes direct acquaintance with the referent of the demonstrative. If you are in the presence of 'this', then pointing to it and uttering "This chair exists" I think should be interpreted as meaning "There is a chair that I am pointing at", in which case there is a bound variable and 'chair' is substituted in for this variable (i.e., 'chair' is in the domain of the quantifier). if the sentence is true, then the chair must exist since the truth of the sentence commits us to the existence of it. Unless, however, there is some way of rephrasing the sentence that gets rid of this commitment, e.g., "There is matter arranged chairwise that I am pointing at." (such a paraphrase might be provided by a mereological nihilist, one who thinks there aren't literally such thing as tables, chairs, etc. but only physical particles or some other such simples arranged in various ways).

  • Can you provide an example demonstrating what "to be is to be the value of a variable" means?
    – asmani
    Commented May 1, 2018 at 7:39
  • @Asmani Good question. See my edit. Commented May 2, 2018 at 12:54
  • The reduction of the natural numbers you've pointed out is only one method that this reduction can be given; in category theory, an alternative foundation to mathematics, it is characterising or universal properties that let us say what natural numbers 'are'; there, any description that fits the properties of a natural number will do: so von Neumanns refuction will suffice, but so will many others. Commented May 2, 2018 at 17:17
  • @MoziburUllah You are absolutely right. Indeed, this 0 = ∅, 1 = {∅}, 2 = {{∅}}, 3 = {{{∅}}}, etc. is a closely related reduction, although not every object involved on the RHS of each equation is identical to objects in the other reduction, which is a crucial observation in Paul Benacerraf's objection to Mathematical Realism/Platonsism, found in his paper "What Numbers Could Not Be."... Commented May 2, 2018 at 19:56
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    @EliBashwinger - Pardon me. I must have misread you.
    – user20253
    Commented May 5, 2018 at 11:42

If you tie existence to perception or measurement, you’ve also de-materialized it — this position is sometimes called “immaterialism”, wherein only percepts and perceivers really “exist”. But what becomes of objects when no one is perceiving them? This aspect is usually resolved by having a universal field of perception (an omniscient observer.) On the other hand, given there are degrees or levels or perception — not to mention the “singularity” of different perspectives — one problem is: how do we handle degrees of “existence” (e.g., objects on the “threshold” of perceptibility)?

  • I carefully chose the word can affect, which is about the potential. So objects still exist when no one is perceiving them. Also by indirectly, I include existence via deduction from perceptions. Hence, the Sun existed 4 billion years ago.
    – asmani
    Commented Apr 27, 2018 at 14:05
  • I don't get your point about levels of perception. I don't think people would disagree about the numbers on the measuring apparatus.
    – asmani
    Commented Apr 27, 2018 at 14:07

I would agree with your "trial criteria," if you change the title to "What are the criteria for physical existence."
Although physical existence is a major part of "existence," there are additional parts. Abstract and/or mental existence.
Although dragons do not physically exist, they do exist as words and symbols in books, in movies, and in human minds.
Nation, city, family, etc., do exist: physically, and as words, and as abstractions.


I think an obvious answer here is to require that anything which exists to have some presence or effect on the world around it.

Physics can help refine the criteria by saying that a thing that exists can be measured (either by practical or not-yet-possible means). Velocity, mass, electromagnetic-force, nuclear force, gravitation. In other words, something that exists has some property.

You add to your question by including ideas or folklore (dragons). You are saying that you think there is some difference here between a physical presence, like an Atom, versus the idea of a Dragon or Government. Dragons probably did not physically exist, that is true. That doesn't mean that the idea of Dragons does not exist, it is encoded in our brains. Dragons exist as a meme in our meatspace. (See talks by Richard Dawkins or Dan Dennett on memes for more complete info).

EDIT1: I mean to clarify the meme comment a little further by saying that ideas only exist in the minds of those exposed to them. When i say the word "Hammer" you conjure up images, feelings, or experiences in the same way that "Dragon" does. The complete implications i will leave to smarter people. Ideas exist in a physical form via reproducible electrochemical signals.

The final argument here being that a physical presence is a property of idea and atoms.


According to W.V. Quine, what exists is what we quantify over in our best theory of the world. (See for example his On What There Is; a classic of analytic philosophy). Or, in slogan form: "to be is to be the value of a variable".

Of course, there are a lot of terms in the above paragraph that need explaining -- quantification, theory, best... -- but that broad approach is a pretty common view across much philosophy in the Anglo-American tradition of the past half century.


I am not clear about the definition of "existence" in this question. If "[s]omething exists if and only if it can affect our senses, either directly or indirectly", then, yes, something exists if and only if it can affect our senses, either directly or indirectly.

But it does not seem like that is what you meant.


Absolutely any idea can affect our senses indirectly. Take enough LSD, and whatever you are thinking about at the moment will have an obvious effect on your senses.. "I am an orange. My hands are jello. Hello. Mr. Unicorn." Good experimental evidence indicates that without the drug's help, all those effects are still there, they are just muted. So this is kind of a useless criterion.

From a POV that takes Wittgenstein and Kuhn seriously, it is counterproductive to define existence. Existence is defined interactively through a language-game, and has obviously changed over time as scientific paradigms have shifted or evolved. You can assert that Quarks always existed, but that is on the basis of a given theory. If we find a different explanation for the effects that caused us to hypothesize Quarks, what happens to their existence? They would then be an apparent epiphenomena of some new scientific object. Is that a way of existing, or a way of not existing?

Given that, what is the point of this boundary? It would surely make future scientific paradigms play silly grammar games. And if it is going to make trouble, it needs a good reason. What would it achieve to set down the rules of what does and does not exist. Would we stop discussing Unicorns, or Angels? Would anyone pay attention to these rules at all?

It may be more useful to take the route of modal realism and to try to sort the order of dependence between different kinds of existence, rather than by putting anything altogether outside the realm of reality. I would point you down the path of an earlier answer What does it mean for something to exist?

  • Only external objects can affect our senses/sensors. We can use our knowledge to determine if something has an external source or not.
    – asmani
    Commented May 1, 2018 at 7:31
  • @Asmani Based on what? Your pronouncement? Extremely important people in the study of sensation completely disagree with you en.wikipedia.org/wiki/Humberto_Maturana
    – user9166
    Commented May 6, 2018 at 2:50
  • Based on what do you know perceptions while using LSD can be false? Based on that.
    – asmani
    Commented May 6, 2018 at 5:59
  • However, I appreciate your point and I admit the criteria is not complete. When I say our senses, I should make it clear that there must be agreement among individuals. This works unless all people take LSD and have the same hallucination.
    – asmani
    Commented May 6, 2018 at 6:02
  • @Asmani Actual experience is not about what "works" it is about what one experiences. So no, I totally reject the idea that what I see, what my actual sensorium actually experiences has anything to do with some negotiated agreement outside myself. You can't just change the question because you don't like the answer.
    – user9166
    Commented May 6, 2018 at 10:49

One possible answer is to turn to Aristotles Categories. The SEP says that this has:

exerted an unparalleled influence on the systems of the greatest philosophers in the Western tradition ranging from his discussion of time and space in the Physics and to the science of Being qua Being in his Metaphysics

This work divides into three parts: the pre-predicamenta, the predicamenta and the post-predicamenta.

In the first part he discusses certain semantic relations, gives a division of being into four parts, and then he lists ten categories; in the second part he discusses in the main four of these categories: substances, relatives, quality and quantity.

His classification rests upon present-in and said-of.

Those beings that are said-of others are universals; and those that are not are particulars; those that are present-in, are accidental; and those that are not, are non-accidental.

Thus, in this classification nations exist as accidental particulars; and dragons do not exist at all.


Yes, all that exists can only known through the senses. There is nothing beyond the senses. The senses are eye,ear, nose ,tongue , body and mind. Mind is special because it enables us to 'see' things which are not directly perceptible. I think that safely defines what existence is.

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    So, there is nothing that exists outside of senses? But then if someone A does not know something X I know, you say that X does not exist for A? But what makes it impossible to say that A are everyone and I don't know about X as well? Then X immediately stops to exist? But then what if there is someome B who knows about X, but no one of A knows about B as well?
    – rus9384
    Commented Apr 27, 2018 at 23:32
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    So it isn't possible for abstract objects to exist, which are causally inert and therefore cannot interact with mind or matter or any other such thing?
    – user193319
    Commented Apr 27, 2018 at 23:50
  • @rus9384 I am saying whatever exists or can exist is through the 6 senses. Knowledge of existence expands through the 6 senses. Having no knowledge doesn't mean no existence. X doesn't stop to exist if we do not know about it. We discover it through our senses. There is nothing outside senses. Coming together of 6 senses defines what is existence. Commented Apr 28, 2018 at 0:45
  • @user193319 Who says abstract object can not exist? They exist through the mind , just as the smell exists through the nose. We interact with abstract object by analyzing them or making use of them in mind. Commented Apr 28, 2018 at 1:00
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    Why are you sure other senses are impossible? I'm not sure.
    – rus9384
    Commented Apr 28, 2018 at 9:10


I am not actually going to directly answer your question, as I cannot speak for modern schools of philosophy. Thus, I will simply explain my personal definition, which (while weird) I think is really interesting and potentially very useful. I hope my theory will be compelling to you, but if not, it should at least be entertaining. (Sorry for the long intro)


I propose that "things" do not exist, I replace the idea of objects existing, with there simply being existence. Existence is the collection of all.

I want to show that: From an objective perspective, there is no such thing as non-existence, it is simply a human construct.

Assume Objects Exist: If we want to talk about objects and objects existing, we need a way to address that. First, we have to talk about the mind/brain. A mind is a piece of the whole for which we have found some set of distinctions from everything else.

What we think of as thoughts are really chain reactions of firing neurons. For the sake of this argument, we will assign mental objects as: Patterns of neural firing that when translated appropriately can resemble objects (collections of existence described by some set of attributes) outside of the mind.

Thus, using this description of mental objects, we can then (from inside the mind) compare objects (see previous definition) outside our mind with the mental objects inside our mind. This is where non-existence comes into play: The only way we can say of an object that it does not exist, is to say that "it does not exist". This requires of us to have some thing that doesn't exist.

Side note, I feel like there was a philosopher who claimed that this means that it is absurd to talk about non existence.

However, there is really an easy explanation for this if we simply talk about it in a relational sense.

If we have some conception of a pattern(object) in our mind, and there is no corresponding pattern(object) outside the mind. We can then say that the thing meant to fill this relational space does not exist. This is the only case where we can say that a thing does not exist.

However, remove the human(or really any conscious) mind, there is then no way to say that something doesn't exist because a metal representation cannot be created, thus, there is nothing to not exist. Thus, non-existence is only a logical concept and cannot be used outside of logical discourse, it can only describe human experience.

Did you have fun?

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