I asked this a few minutes ago on physics.stackexchange and they redirected me here

This question relates to the premise put forth in this TED talk, that reality is nothing but a shared hallucination:


I am totally on board with the idea that our senses often give us a skewed, warped, and possibly wrong idea about what we are perceiving.

My quibble / question is with the "nothing" part of "nothing but a shared hallucination"

For example, take a large fish tank that holds 1000 gallons of water. I understand that a crafty magician could possibly set up a sensatory trick with this. But barring that, my reality is that lifting this fish tank is not something that one human being - even the strongest human in the world - can do alone, given the parameters and variations on the human body.

The mass of this fish tank seems to be an invariant in the universe, no matter what hallucinated perceptions I/you/we have. The mass of this fish tank, it seems to me, is "real". And that's what I don't understand about "reality is nothing but a hallucination"

Can you help me reconcile my dilemma?

  • But if "reality is an hallucination", your perception of the fish tank is "induced" by the magician and there is no real fish tank out there. Dec 12, 2018 at 19:39
  • Of course, the gap between the indeniable fact that perception may be (sometime ? often ?) wrong or not fully reliable to the highly debatable thesis that there is no "reality" out there is a huge one. Dec 12, 2018 at 19:41
  • @MauroALLEGRANZA The difference I see is that if I hallucinate knocking over a 1000 gallon tank on myself, I wouldn't be physically hurt in reality. If I knock over a 1000 gallon tank on myself in the real world, I'd probably be dead Dec 12, 2018 at 20:02
  • @ControlAltDel No, your body is part of the 'hallucination' so it can appear to be hurt. - look up consensual truth.
    – christo183
    Dec 13, 2018 at 5:26
  • 1
    The proper question would be: is perception equal to reality? Classically, yes. But modern approaches define a precise difference. Perhaps my answer here can help you: philosophy.stackexchange.com/a/57718/23407
    – RodolfoAP
    Dec 13, 2018 at 20:29

2 Answers 2


I didn't watch the TED talk, but here are three things about your question.

First, I don't think there's any dilemma here. According to the hallucination argument, there is no mass -- all there is is your sensations: you have a sensation of weight, a sensation of touch, etc., but there's nothing real (according to this argument) beyond those sensations.

Second, here's a way to argue against the claim the everything is a hallucination. The hallucination claim is similar to other skeptical arguments, such as Descartes' dream argument in the Meditations, that everything might be just a dream. Gilbert Ryle famously criticized such arguments:

A country which had no coinage would offer no scope to counterfeiters. There would be nothing for them to manufacture or pass counterfeits of. [...]

In a country where there is a coinage, false coins can be manufactured and passed; and an ordinary citizen, unable to tell which were false and which were genuine coins, might become suspicious of the genuineness of any particular coin that he received. But however general his suspicions might be, there remains one proposition which he cannot entertain, the proposition, namely, that it is possible that all coins are counterfeits. For there must be an answer to the question ‘Counterfeits of what?’ (Dilemmas, pp. 94-95)

According to this argument, it makes no sense to say that everything is a hallucination, since the concept of hallucination presupposes a concept of reality. (Just as the concept of the fake coin presupposes the concept of a genuine one.)

Third, the idea that everything is a shared hallucination doesn't make too much sense. Shared with whom? If it's shared with other people then not everything is a hallucination since there are at least other people.

  • The difference I see is that if I hallucinate knocking over a 1000 gallon tank on myself, I wouldn't be physically hurt in reality. Something I've just hallucinated can't hurt me. But if I knock over a 1000 gallon tank on myself in the real world, that's probably it for me. Or are you arguing that my conscious might go on after death?But what about my senses? How can I perceive anything if I'm dead? Dec 12, 2018 at 20:05
  • @ControlAltDel I am not arguing that we are hallucinating or anything like that; I'm just explaining the skeptic's view (the one arguing for hallucination). The skeptic would say that nothing did hurt you -- there is your sensation of pain, but nothing in reality beyond that sensation. So you're right that something you've hallucinated can't hurt you, but that's in agreement with the skeptic, because according to them there's sensations and nothing else. I think it's a wrong, but that's the view.
    – E...
    Dec 12, 2018 at 20:16
  • The idea is that basically we are all Sophi in Gaarder's "Sophi's World", right? en.wikipedia.org/wiki/Sophie%27s_World Dec 12, 2018 at 20:40
  • If all things we perceive are perceived from the mind: pain, sight, etc. then there really is not much we can prove against it being in our minds. During a dream many times we can be sure we are not dreaming, until we wake up.
    – Robus
    Dec 13, 2018 at 4:36
  • A shared hallucination does make sense as long as not everything is an hallucination. The Perennial view would be that what is not an hallucination (or somesuch) is whatever is 'having' the hallucination. This would be what is real. Thus Bradley's title 'Appearance and Reality'.
    – user20253
    Dec 15, 2018 at 12:30

It is a well-established empirical fact that physical measurements made by two different observers, although not numerically equal, are related by means of well-defined mathematical transformation laws. That is, if we have the measurements of one observer A, and we know its position, orientation and relative motion with respect to a second observer B, we can predict what the observer B would measure if we know what was measured by observer A (I explicitly exclude cases where quantum entanglement plays a central role).

Therefore, if we reject the solipsistic assumption, it seems that there is a very subtle relationship between what is observed by different observers, if it were a collective hallucination we could hardly find such consistency in the measurements of physical phenomena.

This intersubjectivity of physical measurements could therefore be interpreted as an indication that there is an objective reality outside the minds of the observers who observe the physical world (otherwise, it does not seem easy to explain the precise correlations between their physical measurements).

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