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The following quote is found on The Basics of Philosophy page:

Representationalists argue their case from the "epistemological fact" that it is impossible to have experience beyond the sensory surface, from the fact that dreams, hallucinations and visual illusions clearly indicate that the world of experience is not the same thing as the world itself,

What is the difference between the terms "hallucination" and "visual illusion", as used in the quote?

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Please edit your question to provide a little more context. A question that consists of just a snippet of a quote is hard to understand. –  JSBձոգչ Apr 8 '11 at 11:49
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5 Answers 5

up vote 17 down vote accepted

Illusion

  1. Illusion is a kind of wrong perception.

  2. In illusion, an external stimulus is always present. In other words, illusions are caused by external stimulations.

  3. Illusion is almost universal.

  4. Normal persons suffer from illusions.

  5. The same situation arouses the same type of illusion in most people.

Hallucination

  1. Hallucination is a false perception.

  2. In hallucination, no external stimulus is present. Hallucinations are caused by internal stimulations.

  3. Hallucination is a personal experience.

  4. Hallucinations are mostly confined to mentally ill persons and to those people under the influence of drugs. The character of hallucination is determined by the individual's present and previous experiences.

  5. The same situation may not arouse hallucination in all. There are individual differences with regard to hallucination. The same individual may experience different hallucinations are different occasions also.

Source.

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@10Ft: +1 for external vs. internal stimuli. Not sure people suffer from visual illusions however. I'd save that term for those under certain (non-visual) illusions perhaps. –  Callithumpian Apr 8 '11 at 13:03
    
An illusion is seeing a mirage of a sea in the desert (really just the sky reflecting off a heat layer). A hallucination is seeing the same thing when looking at your dog. –  Bill Apr 8 '11 at 19:18
    
@Bill : hehe, well put –  l0Ft Apr 11 '11 at 7:01
    
@Callithumpian : thanks! –  l0Ft Apr 11 '11 at 7:03
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A hallucination is to see something that is not real! For example, if you feel that you are seeing a cat that is flying, then you are experiencing a hallucination.

An illusion is to look at something real but to see it in a different way, like when you look at two horizontal lines but it feels like they are not actually horizontal.

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While the punctuation in the quote is acceptable, placing a comma after the word "hallucinations" would make the sentence a bit clearer (to me, at least.)

...it is impossible to have experience beyond the sensory surface, from the fact that dreams, hallucinations, and visual illusions clearly indicate that the world of experience is not the same thing as the world itself,...

A distinction needs to be made, not only between hallucinations and visual illusions, but also dreams and unmentioned mirages.

Visual Illusions are almost always synonymous with optical illusions. These are tricks of perception--the brain receives information from the eyes and processes that information based on previous experiences. Our brain "sees" what it expects to see rather than what our eyes really see. When there is a difference between what the eyes see and what the brain sees, we call it an optical or visual illusion.

A mirage is a physical phenomenon that happens before the eyes send information to the brain. In this case, the brain sees exactly what the eyes see, so it's not an illusion in the same sense as described in the previous paragraph. It is still a type of optical or visual illusion because ultimately our brain perceives something that is not really there.

A dream is a perception that happens completely in the brain, without stimulus from the eyes. We normally consider dreams only to be those brain-experiences that happen when we are sleeping, but perceived thought-images while 'daydreaming' or in other trance states can also be considered dreams. Again, these are perceptions of things that are not really there.

Hallucinations are brain-experiences that happen to some people when they are fully awake. Unlike visual illusions and mirages, hallucinations are only experienced by individuals.

Visual Illusion:

  • Mostly physical phenomenon, but there is a brain-only aspect to this.
  • Experienced when awake.
  • Experienced by many people at once.

Mirage:

  • Completely physical phenomenon.
  • Experienced when awake.
  • Experienced by many people at once.

Dream:

  • Non-physical phenomenon (does not exist outside of the brain.)
  • Experienced when in a sleep or trance state.
  • Experienced by a single person only.

Hallucination:

  • Non-physical phenomenon (does not exist outside of the brain.)
  • Experienced when awake.
  • Experienced by a single person only.
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+1; If I could favorite an answer, I would favorite this one. –  MrHen Apr 8 '11 at 18:19
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The main difference is the cause.. A hallucination is caused by an altered mental state such as psychosis or drug-use. An illusion is any false perception, with the exception of those caused by altered mental state. Such causes may be deliberate trickery (such as stage magic), optical artefacts (for example certain configurations of shapes), natural phenomena (for example a mirage), etc.

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Both of the terms imply incorrect perception of something. The most basic difference between the two is that hallucination originates within the mind, and illusion originates outside of the mind.

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protected by Mitch May 30 '12 at 2:45

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