Why Idealists Are Not Flummoxed By Perceptual Errors, Hallucinations, and So On

In the comment section of another post, Hume worries how idealism can survive an encounter with illusions, perceptual errors, hallucinations, the movement of the earth despite its apparent lack of movement, and so on.

The idea seems to be that we need some reality independent of experience to correct these errors. We are like children who have filled in tests, and now slip them into a tube in our classroom (the world of experience) and send them off to a reality that is totally outside the world of experience to be graded. Somehow, we know not how, the tests are marked up and returned to us for us to absorb the right answers. This is roughly the correspondence theory of truth: our ideas are true when they "correspond" to some reality that is not ideas at all: although how an idea can "correspond" to something lacking all mental character whatsoever is beyond my comprehension!

Of course, the above is not at all how we actually recognize we were in error about what we saw, or our naive idea of the earth was wrong, or that we are color-blind: what we do is compare some parts of experience against other parts and try to make our world a more coherent world. We see a man hiding in a bush in our backyard. We grab a flashlight and shine it on the bush and he disappears, and we realize "It was just a trick of the lighting." We haven't gone outside the world of experience to correct that world, something impossible: we have brought our experiences into a more coherent whole, by judging "Light does not make human bodies vanish, in my experience, but it does make shadows vanish, so that must have been a shadow." A color-blind person comes to know he / she is color-blind by having the experience of other people differentiating colors which he / she cannot.

When we decide that the earth is in motion, we do so by comparing and weighing different experiences. Think of how Galileo countered the argument, "But if the earth moved we would feel it moving!" He pointed out that we already had the experience of being in motion without feeling it: it happens every time we sail upon still waters. He showed that his idea made our world of experience a more coherent whole, in that it also accounted for our experiences of seeing planetary motion.

We correct mistakes in experience by coming to realize that these mistakes do not fit in with the rest of our experience (men do not vanish when we shine a light upon them). There is no other way we could correct such mistakes!


  1. It is possible to imagine a world where there is something like an objective reality that contains intelligent agents who are part of that objective reality and experience it to various degrees of accuracy via sensory perceptions. (I'm thinking of this world as some sort of sophisticated virtual reality computer program where "objective reality" and "sensory perceptions" are both part of the program.)

    By analyzing their senses and applying logic the agents in this world may conclude that as the only way they experience things is via their senses that the any concept of an outside reality must be false and that idealism is correct.

    But they would be wrong.

    An observer who had insight both into objective reality and their sensory perception of it would be able to validate that their perceptions were indeed "caused" by something independent of their perceptions.

    I conclude that just because we can deduce that objective reality is unknowable it doesn't mean it does not exists. In addition as soon as we include other sentient beings in our idealist world then surely we can conclude that there must be some mechanism (albeit unknowable) that allows our perceptions and their perceptions to be co-ordinated. And that mechanism is indistinguishable from what we might call "material reality".

    1. "the only way they experience things is via their senses..."

      Did I say that?

      'An observer who had insight both into objective reality and their sensory perception of it would be able to validate that their perceptions were indeed "caused" by something independent of their perceptions.'

      So, somehow this fellow got this 'insight'... how, exactly? Not through experience?

      "I conclude that just because we can deduce that objective reality is unknowable it doesn't mean it does not exists."

      Rob, you have TOTALLY mistaken my position. I hold very strongly that objective reality exists. It is what we experience every day of our lives. It is your position that is skeptical, since you think it is unknowable... but somehow we still know about it anyway!

      Rob, both you and I believe that 2 + 2 = 4. That is objectively true. Is there some "mechanism" that coordinates those beliefs in us? Or is it that we live in a world of ideas, some of which are objectively true?

  2. I see now that you believe we experience reality directly, while I indeed think it likely (but never provable) that we actually experience only perceptions of an unknowable external reality.

    In my example of the virtual reality world where both objective reality and perceptions exists the observer has knowledge of how the virtual reality world is programmed and can therefore see (because he understand the code) that the perceptions of the agents are based on something that exists outside of their perceptions.

    Such a world is surely a theoretical possibility and I don't see how we could ever know for sure that we are not living in a version of this world.

    I can certainly see that one could just define "objective reality" as what we perceive (after we have used logic to filter out those things that we deem to be inconsistent and therefor not "real"). But I find to hard to see how one can build a consistent model of a world with multiple sentient beings based on this view. If your perceptions differ from mine do we just conclude that your "objective reality" is different from mine? Or that one of has a better ability to build a more coherent world view than the other (as implied in your post)? If its the latter then that seems close to believing that there must indeed be something outside our perceptions that our personal "objective reality" could be measure against.


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