Suppose that a person can't sense anything. He can't see, hear, feel. Nothing. So for him, does anything even exist? Does that mean only things we can sense are real or existent? Also, how would you define non-existence?

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    When I fall asleep, do you still exist?
    – Schwern
    Commented Sep 7, 2019 at 22:20
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    Descartes already came up with a tidy answer for this. It's weird that we keep revisiting the same fundamental questions. We should come up with new philosophy questions, while sticking to fundamental questions that are universally important to humans. Commented Sep 8, 2019 at 4:22
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    If a tree falls in the forest and one hand hears it clapping, there is no spoon? It is your mind that bends? Commented Sep 8, 2019 at 4:23
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    this question has generated some truly awful answers...
    – user38026
    Commented Sep 9, 2019 at 3:18
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    René Descartes entered a coffee shop in Paris, the barista asked if he would like a coffee, he replied "I think not." - and poof, he disappeared!
    – Glen Yates
    Commented Sep 9, 2019 at 15:58

10 Answers 10


tl;dr- Depends on a person's level of mental development. The truth's crazy complicated, but we go through stages of understanding.

Stage 1: Realism.

The simplest way to understand reality is through the lens of realism. It's the mental model children adopt upon acquiring object permanence.

The gist is that there's one, objective reality that we all share.

Realism is often complimented by classical views of physics. For example, there're 3 spatial dimensions and 1 temporal dimension; things exist locally and concretely; etc..

Ontologically, realism's like Newtonian physics: we know it's wrong, but it's still close enough to correct for many purposes, and so much easier than more rigorous models, that we often use it despite its flaws.

Stage 2: Qualified realism.

Have you ever had a dream that seemed real, then woke up? And if so, how do you know that you're awake right now?

Problems like dreaming break the realist perspective: it's obviously not sensible to assume that we can necessarily access objective reality at any given time since we might be, for example, asleep.

Still, it's not too hard to adapt. We can say that there's still an objective reality out there, and that dreams just show us faked versions of it that might temporarily fool us.

I'd call this perspective "qualified realism": it's not really full-blown realism, but it's not too huge a step from it.

Stage 3: Fuzzy realism.

Did you know that time passes at different rates for different observers in General Relativity? For example, if you drive to the store and then come back, you may've experienced a different amount of time than someone who stayed at home?

Did you know that it's not known to be too meaningful to speak of particular objects as existing at precisely a time/location? There's some variance in there with the uncertainty principle and whatnot.

Did you know that you're getting hit by invisible objects you can't perceive? Neutrinos, and whatnot?

Did you know that you can't really "see" things in front of you directly? I mean, it's actually some light – and that light gets modified as it travels to your eyes, and then that gets turned into a signal that's passed along, etc..

Did you know that the very space you occupy is expanding while you're in it? That your body is snapping back to resist being exploded by it?

If you didn't know some of these things, that's cool – they're not a big deal, practically speaking, most of the time. But the point's that reality's complicated.. so complicated that you can't possible actually know what it is, or even the basic structure of time-and-space, or the types of particles that're interacting with you, etc.. Because it's too darn complex.

If we just ignore it and keep thinking of reality the same way, then we're still basically using realism despite acknowledging that the truth's fuzzier. I'd call this "fuzzy realism".

While realism tends to be complimented by classical views of physics, fuzzy realism is more like semi-classical modeling: classical physics is used in most cases, but we substitute in more rigorous models when we think that the classical model won't do.

For example, a fuzzy realist might tend to perceive time in a Newtonian fashion unless we're talking about stuff traveling on-the-order-of the speed-of-light, at which point they'll accept non-Newtonian models for the sake of calculating perturbations to the background model. The peculiarity in this is that, while a fuzzy-realist might accept that they should use a non-Newtonian model in such cases, they'll tend to feel that the Newtonian model is more "real" while models like from General Relativity will tend to carry an emotional connotation of being somehow un-real.

Step 4: Model-dependent realism.

If you study science far enough, you'll get to points where reality just freakin' breaks. Where you have to acknowledge that a universal reality is too far off to work from, necessitating more pragmatic approaches.

My favorite example is trying to calculate the entropy of a complex chemical system in a reactor. Chemical engineers do this stuff in numerical simulations all the time, but at some point, we have to accept that the numbers we use vary arbitrarily with our model-selections. Or, in physics, it's just really hard to make a solid claim about how far away a star is, or even what a bright point in the sky even is. This Nature article discusses some of the issue with respect to quantum physics.

For example, did you see the recent "picture" of a black hole?

It's not a "picture"! Not in the realist-sense, anyway. Instead, it's a model that we informed with some radio signals.

And you'll agree with me that it's not a picture if you're a realist. But if you've moved past realism, then you'd say that it is a picture – that the method used to photograph it may be non-traditional, but it's still no less "real" than a picture based on a model that's basically a 2-dimensional grid informed more directly by photon emissions.

At some point in a scientist's career, it's no longer practical to be obsessed with this idea that there's a single truth, but rather that there's model-subjective truths.

And that's model-dependent realism.

Step 5: Post-completionism.

While model-dependent realists have moved beyond realism, they still tend to think of their own thoughts as inviolate (or qualifiedly so; for example, they might acknowledge context-specific defects, such as lost memories or confusion due to intoxication), logical systems as complete, etc..

Ditching the assumption that one's mind is an abstraction, rather than material, is weird.

Ditching completeness is weird.

Ditching presumptions of non-orthogonality is weird.

Ditching non-virtualism is weird.

There's a lot of simplifying assumptions that, once dropped, lead to a very complex worldview. These are probably the most common steps after model-dependent realism, but communicating them would be an epic challenge. I can't recall having seen anyone do it well yet. It's my working assumption that no human will manage to aptly discuss these topics before strong-AI is able to mediate the complexity.


To sum up the stages:

  1. Realism:
    There a single, objective reality that we can meaningfully consider.

  2. Qualified realism:
    There a single, objective reality that we can meaningfully consider, however we might sometimes be dreaming or something. Still, once we wake up, it's back to reality.

  3. Fuzzy realism:
    There's still basically a single, objective reality, it's just really complex. Relativity means that there're different pathways through time; quantum mechanics means that things can exist in weirdly delocalized ways; the expansion of space means that space itself is changing through us; there're weird physical phenomena that kinda effect us but we almost can't understand it; etc.. Still, despite our acknowledgement of these various weirdnesses, it's still most productive to think of a single, objective reality, seeing everything else as small perturbations to it.

  4. Model-dependent realism:
    It's become too impractical to keep up with some notion of a single, objective truth. Maybe there's some single, objective truth out there, but it's too far off to be knowable. Instead, we acknowledge models as lenses through which we see the world, and things that we acknowledge to exist exist within the scopes of such models.

  5. Beyond model-dependent realism:
    We abandoned assumptions of mental integrity or/and completeness or/and computational-realism or/and conceptual-non-orthogonality or/and other fundamental concepts. This is a huge, weird space, and this answer doesn't go into it.

Note: Fuzzy realism isn't horrible.

A lot of people still use Newtonian physics even though we know it's wrong. But, it's so freakin' easy compared to more robust models, and it's so very practical in many cases, that it's often sufficient.

Fuzzy realism is similar: it's a cheap, light-weight mental framework that's usually good enough. So, a lot of people stick to it, not having suffered cause to progress to model-dependent realism.

I think this can lead to folks disagreeing about if realism's valid or not. I'd say that realism is an oversimplification, but that it's such a useful one that it's appreciable why many folks would stick with it.

Addressing the question directly.

The above answer focuses on stages of mental development because notions of reality change over the course of one's mental development, such that the answer depends on where a person is at in this process.

Back to the basic question:

Suppose that a person can't sense anything. He can't see, hear, feel. Nothing. So for him, does anything even exist? Does that mean only things we can sense are real or existent? Also, how would you define non-existence?

To go through the perspectives:

  1. Realist perspective:
    Realists assert that there's a universal entity called "reality". Things that're members of reality exist, and things that aren't don't exist. There's no such thing as things existing "for" a person. Your question doesn't really make sense from this perspective.

  2. Qualified realist perspective:
    Qualified realists assert that there's a universal entity called "reality", but acknowledge that there're scenarios in which reality might not be accessible, such as when dreaming. This blind/deaf/etc. person is in such a scenario, where they're not able to connect to reality. So, they're stuck in a state of ignorance, unable to determine what's real.

  3. Fuzzy realist perspective:
    Fuzzy realists acknowledge that it's difficult to know reality in full, but they haven't yet progressed to model-dependent realism yet. If you pushed them to answer your question, then:

    1. If they answer quickly-but-honestly, they're liable to say that they don't really know, as they're fuzzy on some of the particulars about reality.

    2. If they answer quickly but brush off their own uncertainty, then they might answer your question like a qualified-realist would (as discussed above).

    3. If they're hesitant to answer your question due to their own uncertainty, they might take some time to think it out. Thinking it out well enough should lead them to maturing into model-dependent realists (as discussed below).

  4. Model-dependent realist perspective:
    A model-dependent realist would say that the blind/deaf/etc. man would have their own mental models describing what the blind/deaf/etc. man believes. Then, things could exist-or-not within the context of specific models.

In general, we'd say things "exist" if:

  1. they're falsifiable; and

  2. we believe that attempts to falsify them would tend to fail.

The big difference across perspectives is background model which we'd attempt to determine existence within.

Your specific question is relevant in post-completionist models, too, but I'm a bit hurried right now to comment on it yet. (You can try to edit this answer to see the commented-out stuff if you're curious about a rough-draft I started, stressing that it's a rough-draft that I commented out for a reason.)

  • A good opening up of the question. +1
    – Rushi
    Commented Sep 8, 2019 at 4:00
  • @Nat : This is a great answer, thank you ! +1
    – SmootQ
    Commented Sep 8, 2019 at 11:05
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    awful answer, sorry but even the attempt to link different philosophical theories to "mental development" is doomed
    – user38026
    Commented Sep 9, 2019 at 1:45
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    what about the idea that we are living in a virtual reality? some sort of computer simulation. some scientists are considering this. some religions also state that our universe is a creation
    – michael
    Commented Sep 9, 2019 at 10:38
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    Thanks @nat for making the fine finer! Every form(at) has its limit(ation)s. I believe we can push the SE format for a bit though. So instead of making one answer into a "book" it should be possible to add different parts to different questions – grow your book/article organically more than structurally. Eg the earlier q is philosophy.stackexchange.com/q/8384/37256 You could add parts there. This one has my bonus +50 anyways😀
    – Rushi
    Commented Sep 10, 2019 at 4:25

Reality is objective, not subjective. Therefore, you can remove the subjective part of "So for him, does anything even exist?" and make it: "Does anything even exist?" The obvious answer is yes. If that doesn't convince you: That person needs to exist for anything to be subjectively true or false, doesn't it? Therefore (again) true.

"Does that mean only things we can sense are real or existent?" No, see my first sentence.

"Does that mean only things we can sense are real or existent?" I bet I could sense an awful lot that doesn't exist if I merely walked down to the train station, bought whatever I could from shady-looking people, and consumed it.

"Also, how would you define non-existence?" You need a domain D for that question to make sense. A does not exist in D if and only if D = D \ {A}.

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    It's better to consider reality intersubjective. There is no "privileged view" of the universe. It's largely built through an imperfect, but highly effective consensus of individual interpretations. Objectivity is a "simplification".
    – J D
    Commented Sep 8, 2019 at 4:16
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    Subjective idealists would disagree with your first paragraph.
    – user31740
    Commented Sep 8, 2019 at 5:49
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    Lots of people would disagree.with the first sentence. It should be prefaced 'In my opinion'. A common view would be that it is not subjective or objective. .
    – user20253
    Commented Sep 8, 2019 at 11:04
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    Whether people agree or not is irrelevant, as their understandings are subjective; what matters is whether it's true, which is objective.
    – Andrew
    Commented Sep 9, 2019 at 14:52
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    @Andrew Note how he even makes his statement of that your statement being false subjective to him. My guess is that he denies all objective truth (which of course is a really stupid thing to do).
    – UTF-8
    Commented Sep 10, 2019 at 18:45

Suppose that a person ... So for him, does anything even exist?

When you say for him it implies that there is something it is like to be him, if we assume that there is no "for" to a chair or a table.

Let us change "him" to be "chair".

So for the chair, does anything even exist?

Is their a feeling relative to the chair? if there is a for then there is something that is like to be the chair. Which means that at least the chair is aware of its own existence.

So, when you presuppose this for him, you already presuppose that there is something it feels like to be him/her, to be aware of his/her own existence.

But if he/she is deprived of this awareness, then it follows that there is no for to him/her, there is nothing that feels as a for relative to him or her, in fact, this 'person' is an "it" not a "he/she", it is not even a person to begin with.

But since you used these three terms : person, for him, and he, it follows that this entity does not have zero subjectivity (like a dead body), but a subjective inner-world, albeit minimal.

So, to this person, only one thing exists : his/her thoughts.

What makes something real?

This depends on your philosophical view :

If you are an idealist, then nothing exists except this person's thoughts. And if you are a realist, then everything we know of exists.

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    this isn't the best answer; it's not really informative enough. but it's not as bad as the others +1
    – user38026
    Commented Sep 9, 2019 at 3:19

Suppose that a person can't sense anything. He can't see, hear, feel. Nothing. So for him, does anything even exist?

The question only makes sense if the subject is assumed at least partially conscious. I assume that the question is about a human being who is unconscious of the material world but normally conscious of their own mind, so to speak.

We have also to assume that among the range of unconscious states possible, there is a mental state where the subject, while unconscious of the material world, is nonetheless able to form rational thoughts, including thoughts about existence and about what exists.

If we suppose someone merely falling momentarily unconscious at some point, it seems clear that, trivially, their idea of existence and what exists may not change at all from what it was before falling into an unconscious state.

Presumably, in most actual cases, loss of consciousness comes with a loss of the ability of the subject to form any rational thought, but that may not be true in all cases and we have to assume it is not true in some cases at least for the purpose of this question.

Thus, eschewing triviality, we have to assume a subject who either never had any conscious perception of the material world, including of their own body, and therefore wouldn't have formed ideas about existence and what exists prior to becoming unconscious; or would not be able, while unconscious, to remember ideas formed during their prior conscious life.

In the first case, it is anyone's guess whether the subject would nonetheless be able to form a rational thought at all, let alone a rational thought about existence or exiting things. It seems very unlikely.

The second case may be a bit more favourable. However, we may fall back into triviality if we assume that the subject is able to have any memory of the material world, for example what was the subject's last meal, or the fact that the subject watched the news on television the day before.

Presumably, memory of facts about the material world may carry with it the belief in the existence of the things involved in those facts, even if the subject is currently unconscious of the outside world.

Perhaps the crucial point is whether an unconscious subject, remembering nothing of the material world or indeed of their own life, could nonetheless form the idea of their own existence as a mind, so to speak.

I don't think it is possible to answer this question but at least there doesn't seem to be any good reason to assume that it is impossible.

My own experience of being conscious of at least a small part of my own usual mind while completely unconscious of the world outside, including my own body, doesn't support this, though. I think it is to be expected that most cases of unconsciousness would be similarly negative. However, any number of cases somewhat similar to mine wouldn't falsify the possibility of forming a rational thought about existence.

A related question: is it possible to remember one's abstract ideas about the world, ideas about existence in particular, while without any memory of any concrete fact such as that of a meal or of watching the news on television. I would suppose it is, although that would be somewhat infrequent. If so, I don't see why the subject could not think about existence, and in particular about the existence of their own mind.

Does that mean only things we can sense are real or existent?

I don't think so. We still don't know if space exists at all as we usually think of it. We don't sense space as such. We merely infer its existence from the way the sensible world looks to us.

Yet, I don't think many people would be prepared to claim that this proves that space, as we usually think of it, doesn't exist. All we can do is assume that our idea of space is merely a sort of mental map representing what we cannot perceive in itself. And whatever the nature of the thing causing our notion of space, I don't think the idea that space doesn't exist makes much sense.

Also, how would you define non-existence?

As the absence of existence... The hat that is on my head right now doesn't exist. Existence is whatever we think of as existence. I don't think we could possibly tease out anything more meaningful than that.

Reality is usually thought of as whatever exists at a particular time, irrespective of what we may think or perceive. There is, however, no good reason to believe that an unconscious subject couldn't, perhaps only in some very particular cases, form the rational thought of their own existence, as a mind, à la Descartes.


Let's explore this situation by changing our perspective and altering the scenario slightly.

  • If they're blind, does visible light exist?

What if a blind person told you they don't believe in visible light because they've never seen it?

  • If they recover and they can start perceiving things again, do they conclude reality popped back into existence just for them?

Let's say they got hit by a car and fell into a coma. They wake up in a hospital bed weeks later. The last thing they perceived was driving in a car. Do they assume reality stopped existing while they were out and then suddenly created a history when they woke up like some sort of video game?

  • When you see a person in this state, are you a figment?

They can't perceive you, yet you can clearly perceive and affect them and form thoughts and ask questions about the situation. Clearly you're real. Would it be any different if the situation were reversed?

  • When you fall asleep, does everyone else stop existing?
  • When you wake up, do they pop back into existence?
  • When I fall asleep, do you stop existing?

This is the everyday mundane version of what you're asking. At any moment billions of people are asleep, not perceiving reality (or perceiving a different one in their dreams). At the same time billions of people go about their lives.

To answer yes to your question, or any of these questions, we have to accept that one of these billions of people have a privileged place in reality; that only their subjective perception of reality matters. That reality creates itself just for them.

Even in a brain in a jar scenario there is still an objective reality, just not one you are correctly perceiving. There are things all around us we do not perceive and have almost zero practical effect on our daily lives, neutrinos have come up as an example, yet are still (to our best knowledge) real.


Interesting question that raises lot of ethical dilemmas too. If a person is cut off from sensual perception, then are they still accountable for their actions in the world they inhabit? Are they eligible for care? And so on.

But here the question is regarding the 'reality' of the world itself. Let me try to give a perspective about this from the Indian Vedantic tradition, with a story. This happened with a great philosopher-king called Janaka, who fell asleep and dreamt that he had taken part in a war, and fleeing from his defeated army, enters a forest, hungry and exhausted. There he begs for food from forest-dwellers, but just before he could eat the food procured after much effort, some fighting wild boars rush in and smashe his bowl away.

The king cries out in pain and with this he wakes up to find himself in his royal bed and his luxurious palace. But the shocked king has just one question on his lips, "IS THIS REAL, OR IS THAT REAL?"

No one can give a satisfactory answer, till there comes to the palace, a young, deformed boy called Ashtavakra (deformed/knotted in 8 places). He whispers to the distraught king a few words that finally calm him down. Ashtavakra's words were, "Neither is this real, nor is that real. You are real".

The 'you' here, is the king, the consciousness that is aware, whether asleep, dreaming or awake.To extend the analogy, for the one without sensual perception, consciousness exists, because everything is contained in Consciousness. Even inanimate objects exist in Consciousness. Yet, the world as we know it becomes RELEVANT, being built by our perception. When we lose touch with this world (metaphorically too), we continue to exist in it, but the world is not relevant anymore. To some, this might seem like unreality.

To add a footnote, Advaita Vedanta considers creation/world to be an illusion - MAYA. Everything initially is undifferentiated energy, which assumes form to contain creation. It is the senses and the mind, which make it 'real' for individuals, in different degrees. Ultimate transcendence of existence/world happens when we evolve to a consciousness beyond that of the mind. Then we become ONE with the ground of Being. Does the world exist after we reach transcendence? It does. but again, the world is no longer relevant :)

PS: I know this answer raises more questions :)

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    Thank you for the answer. :) Loved it
    – user701763
    Commented Sep 14, 2019 at 8:54

I believe a real existence must be objective and demonstrable empirically or logically. In my opinion, it is not aceptable when I said "something is real for me".


I wonder if this "person" might be bat-like or butterfly-like. Must this "person" be treated as an end and not a means? What is entailed by "personhood?" "For him" or her or it: for is sometimes used in teleological statements. See "Teleology" by Woodfield. Is this "him" then a target which is being aimed at purposefully? Can a person do targeting even if the target is never reached? Is that what it is to "be" human?

In a Psycholinguistics class a story was told about an emperor's experiment to see which language children would naturally speak if they did not hear a language spoken. They were provided with caregivers who were instructed not to speak a foreign language to the children. Of course, the children all died. Even a human foetus needs interaction. Senseless and braindead are not extremely far apart.


The question has to be analysed :

(1) what makes things real vs. mere appearances

(2) what makes things real, that is real substances vs bundles of properties, series of events ( see SEP, " substance")

(3) what makes things real/ actual vs potential entities, possibilities

(4) what makes things real, that is positive entities ( "ens reale", "entia realia" in latin ) vs negations or privations ( like shadows, holes, inabilities, deficiencies)

(5) what makes things real vs " beings of reason, entia rationis " ( objects that are " presented to the mind" without having an actual existence out of it)?

A simple answer is : what is " real" is what can either act, exert an action or undergo an action; activity and passivity are the signs of reality. In this sense only " concrete entities" are real: numbers, sets, universals can neither act nor undergo an action.


for him, does anything even exist?

The flaw in this question is that it was prepended with the phrase "for him."

Things objectively exist if he has five functioning senses.
Things objectively exist if he has zero functioning senses.
Things objectively exist if he once had functioning senses, but lost them.
The things that objectively exist will continue to exist if he regains his senses.
Things will continue to exist even if he ceases to exist altogether.

Things do not exist “for him,” “for me,” or “for you.” They just exist. They exist, even if any given individual (such as you) had never been born. Forgive me, but it’s a bit narcissistic to imply that if you had never been born, there might be some effect on the probability that Kilimanjaro (or Tokyo, or Saturn) exists.

Here is a better question: why did you imagine that the existence of things might be a function of any individual's sensory abilities, when it clearly is not?

Someone else left a comment that the universe is

largely built through an imperfect, but highly effective consensus of individual interpretations.

That’s easily disproven.

For example, exoplanets have existed for billions of years, despite the fact that no individuals had sensed their existence – let alone reached a consensus about their existence – prior to 1991.

  • I guess the counter would be, how do you know that exoplanets have existed for billions of years? It's not too hard to conceive of a simulation that only "draws" the world on demand, popping things into and out of existence as they are observed. I'm not claiming that is the case, but merely showing that your conclusion isn't foregone.
    – JBentley
    Commented Sep 10, 2019 at 8:07

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