Can a person know that something like "free will" must exist even though an exact definition in words, using language, cannot be provided, and in the absence of a complete theory that explains it?

We could also ask the same question about other things, such as consciousness. Can we know that consciousness exists, even if we can't explain or define it? Or an external world: can we know that an external world must exist, even if we can't fully explain or define it? Or God: can we know that God exists, even if we can't explain or define it? Or time: can we know that time exists, even if we can't explain or define it?

Or even our own existence: can we know that we exist, even if we don't know how to (fully) explain or define ourselves (what we actually are)?

More generally, can we know that X exists, even if we can't (fully) explain or define it?

Can we only know those things that we can explain and define using language?

My question is prompted by the recent question Why is the question "Is there free will?", and not, “What is free will?"?.

  • 3
    "Know" feels like a strong term. What about something like "justified in believing"? Can we be justified believing in something that is not well defined?
    – TKoL
    Commented Mar 27 at 12:52
  • 2
    maybe it seems petty, but I think dropping the 'true' part is absolutely a desirable thing there - otherwise, you can't "know" anything unless it's also true, which means you have no idea if the things you think you "know" you do in fact "know". We can measure justifiable belief much better than we can measure 'true', I think - it's much easier to mutually determine if a belief is justified than if it's "true".
    – TKoL
    Commented Mar 27 at 12:56
  • 7
    We can't explain the vast majority of everyday facts/phenomena: hoven, smartphone, cars... Commented Mar 27 at 12:56
  • 1
    maybe add 'time' to the list :)
    – ac15
    Commented Mar 27 at 17:35
  • 3
    We know ball lightning exists due to multiple observation reports, it remains unexplained theoretically. "Exact definition in words" is a misguided idea for empirical phenomena. We want to talk about a real thing even if we cannot pinpoint it, not "defined" thing that may well be a figment. What we want is the entity underlying a cluster of related phenomena, and all we need is some way to point to the phenomena. This is done through what is called "working" or "phenomenological" 'definitions', "exact definitions" are open to future inquiry.
    – Conifold
    Commented Mar 27 at 18:03

15 Answers 15


Gravity is a great example to illustrate that yes, we can be certain a thing exists without having the ability to adequately explain or define it. As with the case of gravity, we can observe it and therefore know it exists. It is our understanding of a thing that limits our ability to define it, and just as we used to explain phenomenon as being "magic" in our early history there are still concepts that seem magical to us today that will perhaps be better explained in the future with our advancements in technological discovery.

  • 5
    While this answer OP litteral question, I don't think ravity is comparable to conciousness or free will. For gravity, while we don't always understand the answer, we have a clear question in mind "why is stuff falling down", "why are things pulled toward each other"... For free will or conciousness, we're not even able to ask a clear question, even less so getting a clear answer.
    – Jemox
    Commented Mar 28 at 12:27
  • We do have to define gravity to say that it exists though. In this case it is defined as "the reason things fall down", or if you are an astronomer "the reason big things fall onto each other". If we were to define gravity differently, such as "the curvature of space surrounding us", most people wouldn't be able to answer whether such a thing is real, or just ramblings of a madman.
    – user369070
    Commented Mar 28 at 22:18
  • @user369070 The best explanations might sound incomprehensible. I reduced SQL Normalization to 4 simple sentences once, but my students still had no idea what it meant.
    – Scott Rowe
    Commented Mar 29 at 1:51
  • 1
    @ScottRowe don't confuse explanations and definitions though: one may need to explain a definition to ensure that everyone uses the same one, but an explanation is not definition. Definition is the matter of argument, word is the method of reference to definition in natural language, explanation is an implementation detail - a carrier for matching words (and more!) to definitions.
    – user369070
    Commented Mar 29 at 4:16
  • 1
    The reasoning here is sound. An equivalent example is: Fred. I know Fred exists. I recognize him when I see him. But I can't completely define him or explain him. I just know him when I see him. Commented Mar 29 at 18:54

You need at least some definition, but it doesn't have to be exact or detailed.

You can't tell me whether "adfgiuadhfg" exists, because you don't know a definition of that word.

A child can tell you whether "water" exists, even if they don't know its chemical composition. (Some dictionaries include its chemical composition in the definition, some don't.)

A layperson 1000 years ago could tell you whether "sun" exists, even though their definition for it was just "a bright thing in the sky".

Of course, when you conclude that X does or doesn't exist, it only applies to that single defintion. Proving that there's a bright thing in the sky is different from proving that it's a flaming ball of gas.


For something to exist, it needs to be observable, either directly or indirectly. Physical objects and phenomena can be observed directly and measured. Abstract ideas can be observed indirectly by observing their effects on people's behaviour.

You don't need to explain anything to know that it exists.

But you must have a definition. Otherwise you would not know what is this thing you should be observing.

  • On the need for definitions: observations show that galaxies rotate at speeds that do not fit our current theories. Possible explanations have been proposed, but they tend to fall into one of two seemingly incompatible camps: either dark matter or modified gravity. Both of these camps have significant problems to be resolved. Thus, we can define the problem, and we are looking for an explanation, but we do not even know if what's missing from our current knowledge is a thing of some sort.
    – sdenham
    Commented Mar 28 at 12:06
  • @sdenham Dark matter is a really good example here. To answer the question 'does dark matter exist' we do need to define what we mean by 'dark matter'. If we define 'dark matter' to be a gravitational anomaly based on the assumption that general relativity is correct, the answer is an unequivocal 'yes, it exists'. However, if we say dark matter is WIMPS, the answer is probably 'no'. I think the reality is that 'dark matter' is the former which is why most scientists think the question to answer is 'what is dark matter?' and not 'does dark matter exist?'
    – JimmyJames
    Commented Mar 28 at 21:10
  • @JimmyJames The difficulty here is that the phrase 'a gravitational anomaly based on the assumption that general relativity is correct' defines the perceived problem, not whatever it is that accounts for this perception. Furthermore, 'dark matter' would not apply to any solution to the problem; it would only apply if that solution is matter which interacts gravitationally. The answer to the question 'what is dark matter' depends strongly on whether the anomaly results from some form of matter; if it does not, then 'dark matter' is defined as a failed hypothesis, not as something.
    – sdenham
    Commented Mar 29 at 14:14
  • @sdenham "Furthermore, 'dark matter' would not apply to any solution to the problem; it would only apply if that solution is matter which interacts gravitationally." Only if we are very literal about the words that make up the term. A 'koala bear' is not a bear but term is still how we refer to that animal. We talk about electron 'spin' even though it doesn't spin in the conventional meaning of the term. 'Strange' quarks are not any stranger than other quarks.
    – JimmyJames
    Commented Mar 29 at 15:17
  • @JimmyJames I don't think your analogies are applicable here. The terms 'dark matter' and MOND were coined because physicists found it useful to distinguish between two very different approaches to solving the problem presented by the observations. If MOND turns out to be correct, and yet it would be excessively literal to say that dark matter was not the solution, then it would equally be excessively literal to make a distinction between them today or yesterday - but this is clearly not the case.
    – sdenham
    Commented Mar 29 at 17:14

If we can discuss something, and the participants in the discussion have similar understandings of that thing, then it clearly exists in some ontological sense. We don't have to be able to explain it in detail; indeed, in the case of the concepts you listed, the whole point of these discussions may be to come up with the definition. If it doesn't exist, what are we trying to explain?

In fact, this problem comes up for practically everything, not just weird abstract concepts like consciousness and free will. It's rare that we can come up with definitions of categories that precisely define all and only its members. If you have a definition of "dog", you'll likely always be able to find exceptions that are still considered to be dogs (e.g. the definition might include being four-legged, but there are many dogs with birth defects or injuries that result in loss of a leg). Yet we all intuitively know what dogs are, and we believe that we're mostly in agreement.

So this is just a matter of degree, not kind. We intuitively have shared understandings of what it feels like to be conscious and have free will. But since these are not directly observable like dogs, it's harder to translate these understandings into definitions and explanations. But there's some hope from modern technology. If you're a materialist, I think you believe these are the direct result of neural processes in the brain, and our ability to examine the brain's activity has been steadily improving in recent decades. Finding the "neural correlates of consciousness" may ultimate provide the definitions we're seeking.


Perhaps this is a frame challenge, but I think the real question posed by the question you link to: "Why is it "is there free will?" and not "what is free will?" isn't so really about whether we can explain or define free will, but whether we know what question we are trying to answer. This overlaps with the concept of 'definition' but I think 'definition' might mean slightly different things to different people and/or depends on the context.

For example, if I ask: "do flurgleths exist?", I think it's obvious that the question can't be answered unless we have some definition of 'flurgleth'. We can answer the question: do unicorns exist (no) and kraken exist (yes) because we know what those words mean.

I gave the example of lightning in the comments. Does lightning exist? Yes and we have a precise definition of what it is and a pretty good idea about what causes it. But what about in ancient times? Can you define lightning without an understanding of electricity? I think the answer is 'yes' you can define it without understanding it precisely. There are properties of lightning that can be objectively described without understanding the mechanisms behind it.

This brings us to 'free will'. Can we define it? Perhaps. Do we have an objective definition that everyone agrees to? I don't think we do.


The question has about as many answers as there are philosophers. What exists (ontology) and how we know that (epistemology) make up the bulk of metaphysics. Hobbes had a good materialist definition of knowing or at least "understanding" as the ability to make or reproduce something.

This lingers on in an empirical ontology comprised only of those sense data we can describe, ideally in reproducible equations. Prior to Einstein the existence of atoms was hotly debated and Mach never did admit they "really existed." Boltzmann, on the other hand, "knew" they existed because of a mathematical necessity in thermodynamics.

In some Wittgensteinian 2.0 sense nothing we really know can be described, explained, or "spoken of." All of of true import is lost in translation, so to speak. Yet in an epistemological sense we have no proper business "talking about" things outside of the reach of language, math, or communicative structures.

We can all say we know things that exist as the objects of our knowing and can't put into words. That's fine, it's just not science or persuasive philosophy, it's the ineffable pronouncements of preacherss, romantics, visionaries, and overacting thespians. Things that exist and are known to exist but cannot in any way be described are generally and quite literally called "meaningless."

You may know deep down your "better self" is there or you had an indescribable dream last night, I may nod tenderly but why should I care?

  • You had a dream that was profoundly meaningful to you. Ok so its meaningless (more correctly incommunicable) to me. How did it stop being meaningful to you? More precisely when did it become meaningless to you? After I meaningfully(!) communicated to you that it was meaningless?
    – Rushi
    Commented Mar 27 at 18:47
  • Not sure I follow. If dreamer can't describe the dream, which happens, we suppose, it simply can't have "meaning" for others or "meaning" at all, according to some views. Commented Mar 27 at 21:16
  • 2
    It has meaning for the dreamer. By what view is that meaning negotiable? Well... I guess by behaviorist or some extreme versions of logical positivism? The logical limit of that is qualia. Every qualia of yours in inexpressible to others — by definition. Every single thing that you express, communicate and share objective meaning about will have its layer, however thin, of qualia. See that red apple on the table? We can point to it, share it. But its redness, its taste is your and my qualia, even the table, its hardness, shape.... All incommunicable. Ultimately meaning itself is meaningless
    – Rushi
    Commented Mar 28 at 1:38
  • This happened more extremally when Leary made LSD experiments. The persons who were 'LSD-fied' often described later beauty so extreme they no colors-words, no shape-words just no words
    – Rushi
    Commented Mar 28 at 1:47
  • 1
    Tnx for the exchange. It prompted my (short) answer on LSD
    – Rushi
    Commented Mar 28 at 2:04

As a thought experiment, consider the case of an average laborer from ancient Egypt who gets transported through time to the present. He suddenly finds himself in a control room watching the SpaceX crew launch a rocket into orbit. When he subsequently gets returned to his time, he would be completely unable to meaningfully describe or explain anything that he saw. He knows that the rocket really existed since he witnessed it himself. His inability to describe it is both a failure of language (no words yet exist that come close to an accurate description) and of context (even with the right words, there's a lot of scientific background knowledge that needs to exist before you can explain the rocket). Neither of these things has any relation to whether or not the the rocket actually existed.

To look at it another way, language is a distinctly human thing. Animals communicate, but don't use anything remotely complex enough to be considered language. If your supposition were true, then it would be impossible for an animal to ever actually know anything. I find that conclusion difficult to believe.

  • Insightful thought experiment. But how would you apply this to the specific examples I list in the question?
    – Mark
    Commented Mar 27 at 22:57
  • 1
    Counterpoint: What he knows exist is an image of a tube that expells flames and flies up. He knows not of anything regarding a rocket.
    – user369070
    Commented Mar 28 at 6:40
  • 2
    The egyptian would say there was a giant arrow with fire on its tail going up into the sky. Or more likely he saw something moving inside a window that one could not stick your hand through while surrounded by strange people in strange clothes all of them staring into these windows. But what one saw in the windows, are not on the other side of the windows. Commented Mar 28 at 12:07
  • Up Goer Five
    – Scott Rowe
    Commented Mar 29 at 1:28
  • @user369070 True, you could cobble together some sort of description, but that's nowhere close to the level of explanation that the question is asking about.
    – bta
    Commented Mar 29 at 18:23

Many things exists, and we can't explain or know how it works. Like computers, many don't know how they are programed, how this website is programed, but we still accept that it exist. It may be a survival instinct, to just accept what we are taught, without a full explanation. "This is how you work a computer, but we won't tell you how it works". If we spent a long time pondering how everything works, we wouldn't be able to get much done. So, yes, I think we can know many things without explaining it.

Additionally, there are not just objects that we can't explain, we also have many abstract things such as love, hate, art. What is art? This is a question that leaves us thinking.

Animals don't know language, but do feel emotions that they themselves can't explain, such as a monkey mourning over her dead son.


First you need to do away with natural language: forget the words "free will", and ask whether the concept "" exists, and you should see that the definition of a concept and the concept itself are the same thing. The definition obviously doesn't have to be in terms of natural language: "a perception that evokes a certain specific feeling in you" is a valid definition. Many people confuse rigorous definitions with "whatever I feel like matches it", which is the source of your problem here. Do note that defining words is not a problem of philosophy: its a problem of usefulness in a context, maybe sometimes linguistics. As such different questions of whether free will exists will yield different answers: usually the answer is: "the question is ill formed". Sometimes the answer is yes or no.

TL;DR: No, you can't say whether something exists if you can't even define it. You can't even form a question to ask that.

  • Explaining a definition is not necessary though: some things can be assumed to be 'a priori', or 'a local variable' or 'for the sake of the question itself', or even 'i don't care/don't have time to care'.
    – user369070
    Commented Mar 28 at 6:53

No, but...

Your first problem here is that several of your examples are not an object that exists. They're processes, not objects. Your second problem is that you're grouping things together which do not necessarily belong together.

For objects, we can inspect them with various degrees of accuracy, and determine their state with whatever level of accuracy the measurement method allows.

Processes are a bit different though. Processes are a description of the behaviour of an object and how it interacts with other objects. You have to watch how an object behaves, and then try to fit some kind of model to it which will predict the object's behaviour in future. There isn't any requirement to think up a reason for why this happens, at least initially. The first requirement is simply to find a model which fits past and future measurements. After that, you can start trying to fit explanations to that model.

What tends to happen is that we find classical models are generally good enough for most purposes. It's only when we dig into the more obscure corners that things break down. All your examples are top-level classical concepts. They all break down in various details, so we can't in any way say they're "true", because we know ways in which they aren't - but at the top level they're just good enough for day-to-day use if you don't look too closely.

"Free will" is a great example. You can hypothesize all you like about quantum states in the brain, and whether we're truly deterministic creatures. But even at the macro level, magicians can "force" choices (or what feels like choices) from participants which actually are predetermined by the magician; or advertising and social pressure can prime us to buy things we might not otherwise "want"; or social pressure (and the fear of retribution) can force us to do things we would not want to do. These methods of hacking our perceptions of free will are so well understood that there are literally books about how to do it. Free will is a "good enough" model of how people behave on their own; but the exceptions also need to be considered when modelling how groups of people behave together.

Time is another interesting one. For classical purposes, time is fixed. But as you get faster and faster, the difference becomes significant. This isn't just abstract theory - it's essential if you want your GPS satellites to get you the right position.

God is the exception here. It looks like an object - but in fact it's a place to group together all those processes we don't (or didn't) understand. God has been (and still is) used as the reason people die of diseases, or why lightning strikes buildings, or avalanches or extreme weather or earthquakes or tsunamis happen. As science has advanced, the "god of the gaps" principle has taken all these processes away from a hypothetical god, because the idea that they're caused by (a) god doesn't actually give us a useful way to predict and mitigate them. Sacrificing doves isn't a great medical intervention, and it doesn't stop lightning strikes!

Of course a god or gods can also give us the idea that there's a life after death, which these days is their primary role. Is that real, or is it merely the process of a group of people inventing a comforting fiction in a world where they don't understand all the things which can apparently kill them for no reason? There's no evidence either way. And still, this is always (a) god as the representative of the process of dying, not as an entity in its own right. We know dying happens as a process, and religion is a way you can model it in a way that results in you feeling better about it. That's not necessarily a bad outcome, so long as it doesn't go further than that.


Here is footage from the 1950s experiments with LSD.

The lady ends with If you cant see it I cant tell you about it... I feel sorry for you.

Its your choice and call ultimately, which view you give priority to:

  • She's been given a hallucinogenic; she's hallucinating. What more to say? — the commonsense view
  • She saw beauty so extreme it was indescribable — the lady's first person view

Added later

While the above may suggest that words are unimportant and below in comments I say that, I think this should be refined a bit. Here is a traditional account of 4 levels of communication — where at the lowest level its just verbiage and at the highest there are no words at all.

  • 1
    "I can see it. Can you see it? It is beautiful..." Experience can't be reduced to a travelog.
    – Scott Rowe
    Commented Mar 28 at 11:24
  • 1
    @ScottRowe Yes! You are interested in eating the mango but instead spend your time counting the leaves of the tree Ramakrishna We have forgotten how pointless words are for anything that really matters
    – Rushi
    Commented Mar 28 at 11:28
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    Yeah, animals seem to have the edge on us here, not getting caught up in musing.
    – Scott Rowe
    Commented Mar 28 at 11:30
  • 1
    @ScottRowe First practice love on animals; they react better and more sensitively — Gurdjieff. But I think the real point he's making is they can teach us to love without words. You may have seen stuff on dolphins, horses and ofc dogs being good to "differently abled" children
    – Rushi
    Commented Mar 28 at 11:38
  • 1
    Our conversation above prompted a small Added later. Tnx @ScottRowe
    – Rushi
    Commented Mar 28 at 11:46

We "know" something is out there when we have models (based on mathematics) for it and these models not only explain how this phenomenon works but we can also predict its future state.

Since these models are currently incomplete ( https://en.wikipedia.org/wiki/Theory_of_everything ) and created by human beings who don't have infinite knowledge and intelligence, there are some things which we observe and for which our theories just don't work. I'm not sure whether I've ever heard scientists use the term "know", e.g. in regard to dark matter or dark energy. Normally they say, "there's something out there which is not accounted for", i.e. for which our theories do not work and we need new physics theories to understand and explain these phenomena.

Speaking of "consciousness", some philosophers claim there are very peculiar things about it which science doesn't/can't explain but that doesn't mean we "know" it exists.

"Free will" is an even worse term because there's no phenomenon which needs "free will" to exist.

In the end, yes, we can claim "we know something exists" because we can observe/infer it directly or indirectly and our existing theories don't explain how this thing is possible. And once we have the theory about the thing we thought we knew existed it might as well cease to exist.

E.g. people in the relatively recent past thought lightning bolts were cast by God(s). Turns out lightning bolts are real, and exist, yet the Gods who cast them don't need to apply.

  • ""Free will" is an even worse term because there's no phenomenon which needs "free will" to exist." I'm going to add this to my toolbox about a deterministic universe.
    – DKNguyen
    Commented Mar 28 at 0:25

These are all very complicated question that have no clear answer. The concept of Existence is difficult. There are different streams of philosophers - some allow concepts to exist, some do not. Some call "Existence" an attribute of an object, some do not.

It is not clear what "know" means. Knowledge is a wide field, both regarding how it is defined, and what can be known.

As to your particular question: "free will" would probably be considered a concept by most people; and "exist" would probably not include such concepts for most. So "free will" does not "exist". But an actor could "have" free will, i.e., free will could be an attribute. (Or probably many other ways to combobulate these terms).

Conscience is a special case as it mixes up the observer with the observation. You're probably familiar with the quote "I think, therefore I am", and in this sense many probably would say that conscience is the only thing that we can be sure exists (at least in the singular - i.e. I can be sure that my conscience exists - I don't know about yours).

"God" is complicated because it is not necessarily a concept (if you are a believer). Someone who believes will tell you that God exists; someone who doesn't, won't.

  • 1
    Consciousness rather than 'conscience'?
    – Scott Rowe
    Commented Mar 28 at 23:00

For a real-world example, consider the duck-billed platypus which just happened to exist, though most 'experts' said that because they could not explain or define it, it must be a fake.

Suppose either of us sees an unidentified object. As an extreme example, an unidentified flying object.

We know it exists because we've at least seen it and perhaps also heard or smelt, touched or even tasted it.

How could our complete failure to explain or define it mean that it didn't exist?

  • Free will: It is not controversial that we humans feel free in most situations. We have the impression to be free, not only in our doing, but also in our volition, and most of all in our decisions.

    On the other hand, we do not have an explanation wich combines the phemomenon of free will with the basic principles of our natural laws of nature, which act as causal deterministic laws.

  • Consciousness: The same situation holds for conscious. We feel conscious most of the day, but we cannot explain qualia and other conscious perceptions. Nevetherless neuroscience investigates with some success the neuronal correlates of consciousness (NCC).

  • External world: The assumption of an external world is the most simple explanation for the common view and agreement of humans about the similarity of our perceptions. The objection of solipsism is possible, but it is hard to live and to act consequently on the basis of solipsism.

  • God: The interpretation of certain individual phenomena as religious phenomena, triggered by a personal God, is controversial. The discussion between theists and atheists continues since thousands of years. The echo resounds also on this philosophical platfom.

    Placing side by side all proposals for a definition of God shows, that there is no general accepted definition of God and no agreement about the existence of God.

  • Time: Humans perceive time in a subjective manner, which is not identical with the view of science. Since the Theory of Relativity, physics considers time as one dimension in a 4-dimensional continuum.

    Time cannot be separated from space. Both are intimately linked in
    the physical concept of spacetime. In separation, neither time nor
    space have absolute meaning. Instead the splitting of spacetimes
    depends on the chosen coordinate system of the observer.

    In daily life it is good practice to measure time by clocks.

All of these concepts can be further problematized in philosophically way. But wit the exception of the God-concept and the issue of free will, in order to deal with these subtle questions is not necessary for the daily life and the orientation in our living space.

In every case one has to discriminate between the phenomena, a working definition of the phenomena, and the explanation of the mechanism behind the phenomena. In most cases, one operates with these phenomena on the basis of a working definition.

Without the first two steps it makes no sense to start with the final step of an explanation: If I don’t know what I am searching, I cannot decide whether I am successful in finding.

But it is not unusual that a successful investigation improves the initial working definition.

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