I know this question has been asked before, so before you shut it down for being a duplicate, please hear me out. The answers I saw in other similar threads were not satisfactory to me and they left me with more questions than clarifications, mainly because most of the highly rated answers seemed to miss the following point:

People assert that dreams -- including lucid dreams -- are noticeably less vivid than our waking state. But isn't this argument roundabout? If the question is "how do you know that what you currently perceive as reality is not in fact a dream world", then how can you go about arguing "well, dream worlds are not as vivid as the real world"? This misses the point entirely.

We've read Descartes' first Meditations, as well as John Pollock's A Brain in a Vat, in our intro to philosophy course and I'm really intrigued by this subject. One of the things Descartes mentions is how there is really no way to distinguish the dream world from the waking state, or that everything in dreams is an extension of things we have seen in the external world.

Is it possible, then, to prove that one is not, at any given point in time, dreaming?


4 Answers 4


From the point of view of very aggressive modern theories of perceptual modeling, say something like what is reviewed here you are always dreaming.

Rick Strassman emphasizes that dimethyltryptamine, the drug that immediately induces waking dreams, is naturally produced in the body, and we metabolize it in small quantities during ordinary waking life all the time. Its consumption is increased by tasks like 3D processing and guided visualization.

Since our sensory organs have no built in stability, we compensate for our own movement and scanning behaviors through visualization. From that point of view, the difference is just in how much external stimuli shape the layer of dream projection that enables your perception of stability in your environment.

Theoretically, that scale is continuous. During what are clearly dreams, when you are paralyzed in sleep, external reality is drowned out by internal 'noise'. And during full waking, you are incorporating as much external reality as you can accommodate at any given moment. But we are often somewhere in between. So, this approach would say that when I visualize a complex 3D shape, or check my work against the memory of a class diagram that isn't actually drawn on anything, I am dreaming on purpose, even though I am awake.

That turns your question into a 'sorites' problem, what is the clear line between points on a continuum?

  • Absolutely, the undoing of all the "how do I know I'm not dreaming" questions is the false (or at the very least unverified) assumption that dreaming is a categorical state that one could either be or not be, which, as you've clearly pointed out, is not well justified. To add to the answer though, I would suggest including the experience of Schizophrenics, who, in their waking life regularly experience sensations which seem real to them but which do not concur with the waking experience of the mentally sane.
    – user22791
    Commented Jan 7, 2017 at 7:37
  • @Isaacson It is definitely relevant, and perhaps a route to same answer, but I am not sure mixing the two would improve either. I have both worked with schizophrenic clients, and tripped enough to develop opinions on the subject. I think projection (occasioned by dreaming. 'astral' projection, or sensory deprivation), real dissociation, and psychosis are largely independent starting points that necessarily lead into one another, rather than actually being parts of a single related phenomenon.
    – user9166
    Commented Jan 8, 2017 at 18:46
  • If I added one, I would feel compelled to add the other, and then I would get all tied up in nuance before really answering the question.
    – user9166
    Commented Jan 8, 2017 at 18:46
  • Fair point, I see where you're coming from, demonstrating the something previously considered binomial is, in fact, a continuum could prove a never-ending task if one were to do so by trying to provide all the examples along such a spectrum.
    – user22791
    Commented Jan 9, 2017 at 7:24
  • @Isaacson Actually, in this case, by my reckoning, there are at least three dimensions involved, and not a single spectrum. You make up a good part of your perception, and that is the part I consider dreaming. You actively force your beliefs on your perception, creating impressions that are simply not real, and that is kind of the schizophrenic dimension. And you filter the input according to what you are prepared to accept, which I see as the dissociative dimension. Fascinating, but more complicated than an answer needs to be.
    – user9166
    Commented Jan 10, 2017 at 3:18

According to Bertrand Russell in his book Human knowledge one cannot prove one is not dreaming:

It may be said that, though when dreaming I may think that I am awake, when I wake up I know that I am awake. But I do not see how we are to have any such certainty; I have frequently dreamt that I woke up; in fact once, after ether, I dreamt it about a hundred times in the course of one dream. We condemn dreams, in fact, because they do not fit into a proper context, but this argument can be made inconclusive, as in Calderon's play, La Vida es Sueño (Life Is a Dream). I do not believe that I am now dreaming, but I cannot prove that I am not. I am, however, quite certain that I am having certain experiences, whether they be those of a dream or those of waking life .


No, you can't know for sure that you aren't dreaming, or at least are in some kind of simulation. Even if the world is real, you are still a "brain in a vat", just a vat called a skull with your brain connected to your body. The same way your body sends information to your brain about the state of the world, a machine hypothetically could send identical information about the state of a virtual world. The means to do this may be beyond us for now, but it is completely possible in principle.

You can't argue out of this, either, except pragmatically. Accepting that the world is not real is impractical. If it's not real, there doesn't seem to be anything we can do about it. The best we can do is live as if it were real. Once we realize that we just won't be able to figure this out, we can at least assume the world is real and move on.

Another thing to notice is that the dream problem is also about how we justify our reason. I have been convinced many times in a dream that the dream world was real. I would even walk up to a table and knock on it, or pick up an object and look at it, and declare that it was obvious that everything was real. Upon waking it was clear how wrong I had been. But what else could I have done? I had "evidence" that my world was real, or so I thought. I believed I rationally and reasonably came to the conclusion that the world was real. If "knowing" something ultimately boils down to an emotional state, then how do I "know that know"? If something can just give me the feeling of knowing, how do I know that that feeling is phony or justified? How do you justify reason if you have to use reason to do it?


AleksandrH, the distinction of dreaming and waking, whether either is vivid or otherwise, is, of course, something you could likewise be dreaming... In short, if you cannot distinguish "between sleep and wake" then the answer is simply, "no". I find it very doubtful, however, that you are incapable of telling the difference (by which I mean literally: "use your words").

Consider, however, that a proof is just a convincing argument. If you are looking for certainty then consider that certainty is just a mood. Doubt, whether in the form of an evil genie deceiving us or hallucinatory permanence, is an inevitable consequence of conscious consciousness. Note that tho doubt may have been a useful tool to DesCartes in concluding cogito ergo sum he did not conclude dubito ergo sum.

How then to address the doubt of external realism?

The argument "I may be dreaming" is senseless for this reason: if I am dreaming, this remark is being dreamed as well and indeed it is also being dreamed that these words have any meaning.

-L. Wittgenstein
On Certainty #383

If everyone were dreaming or deceived or hallucinating (or just simply lying) all the time, how then would we have come to have language?

Are doubt and uncertainty adequately reasoned or a proof that the external world* is not as it appears to us when we are awake?

Consider the difference between the "veridical" case that you are verifying your friends claim "it is raining" and the one where you are dreaming that you are indoors with your friend, they have stated, "it is raining" and in your dream you go outside to verify the claim. In the both cases you are aware of something. In the former, the world, in the latter, your dream. The more experience you have with observing your dreams, whether lucidly or otherwise, the better you will be able to distinguish the two. When you have woken up, and let's assume it is actually raining, are you unaware of a difference in what has been observed while asleep and while awake?

  • see chapters 7 and 8

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