A simple error or a mistake isn't always an illusion or hallucination or delusion. You can be tired and adding a series of numbers and make a mistake. Or you can hear it will rain today and believe it and be wrong.

An example of an illusion is when you see a piece of wood and it looks like a dog or bird. This is more than a mistake: Here there is a flash where you see it the way you always see it when you are see correctly, but, in this case, you turn out to be wrong. It's stronger than a mistake; it's not just a belief that turns out to be wrong instead of right; it's a truth that turns out to be wrong.

Illusions, according to the traditional definition in Western philosophy and psychology, has a stimulus that is mistaken for something else. There is the piece of wood that looks like a dog. Hallucinations, on the other hand, by the traditional definition, do not have stimuli. You are walking along and hear a voice talking to you. It's not the wind that sounds like a whisper. There is no sound, but you hear a voice.

Often many people have the same illusion in the same situation. Many people passing the piece of piece of wood might see a dog. Hallucinations are usually unique to one person. Illusions can be unique to you, but they can be more general, even public. Magicians create illusions in us to make us "see" things disappear and re-appear, and so on. Magic — the kind children learn from magic books — is the "Art of Illusion" not the "Art of Hallucination."

Dreams have been thought of as sleeping hallucinations, and hallucinations have been thought of as waking dreams.

By the traditional psychological definition, a Delusion is a persistent and false belief, insisted on in spite of evidence that convinces most everybody else that the belief is false. Illusions and hallucinations come from the senses. The eyes or the ears. There are, as is said, visual and auditory hallucinations. There are even hallucinations of smell. A woman I treated once smelled marijuana where she was living even though no one at her home smoked marijuana. She was terrified the police would come and arrest her.

Illusions and hallucinations have a sensory side, but delusions do not require the eyes or ears or nose. You can believe you are God or that you can fly or that you are president of the United States or that you have an hundred gold mines and an hundred diamond mines. These are thoughts or beliefs that may involve the imagination but do not involve the senses in a direct way.

Obviously, one persons reality can be another person's hallucination or delusion. A woman sees beautiful angels hovering over her sick child, and her husband thinks it's an hallucination and that his wife is simple-minded. The long tunnel with a light at the end reported by many people who have had near death experiences, is seen as an hallucination by some and as the reality of where we go when we die by others. 

This type of debate can be more sinister when a man reports he hears a voice telling him he must shoot his co-workers or when a woman becomes convinced that life is awful, that there is a better world waiting for us, and she gets her five kids into her car and drives off a cliff into the ocean to take them all to heaven.

It is apparent, or should be apparent, to all of us why persistent hallucinations and delusions are important or all of us, why they are often considered a sign of mental illness, and why people with certain experiences, no matter how real they seem to the persons themselves, are locked away in secure mental hospitals by those who think the experiences are hallucinatory or delusional.

A slightly different take on the above:

I have found it useful to change the definitions given above, just slightly. I keep the usual idea of "simple error" or "mistake," but I tinker with the other three concepts. I accept the idea that an illusion is more than a mistake: It involves a "flash" of seeing something — an actual experience of the dog and not just a mistake about there being a dog there. But, I think it is useful, to think of illusions as covering inner flashes, inner perceptions, as well as outer ones. You can glimpse a stick and see a snake (an illusion of a snake), but you can glimpse a truth about your spouse or friend or about the state of the economy, and I call this an "illusion" as well. There is no visual perception with the eyes, but there is an inner perception with the understanding.

Just because you get a visual flash of a snake by the road, it doesn't mean there is a snake: When you look again you see it is a stick. In the exact same way, just because you get an inner flash that your friend slighted you or that your wife is having an affair or that Apple Computer stock is bound to fall, it doesn't mean you are right: When you think about it later, the "illumination" may disappear, and then you wonder why you thought such things.

As for delusions, I think it makes sense to move from the idea that delusions and hallucinations are separate but equal, one being errors of thought and the other being errors of sensation. I think it is clearer to think of delusions as being a step beyond hallucinations. Hallucinations can be via sensation or via thoughts: They are sudden outer or inner flashes or perceptions or an understanding of a something real, but this something real turns out, later, to be unreal. A delusion is one step further along: It is a seriousness about the hallucination, a commitment to its reality, a refusal to back down in the face of competing evidence.

An hallucination can be fun and casual. A person can take a recreational drug for the purposes of having illusions and hallucinations and to escape from reality. The arts are a form of illusion that we seek out, but hardly anyone takes them as literally true after leaving the theater or museum and only barely while in the theater. Standing in front of a statue from ancient Egypt, for a second we may be transported back to Egypt of millennia ago, but we don't think we really went back. In a movie, our heart beats faster at a scary part, we may even scream out, so, in a way, we do take it as true for a second or a moment, but it only becomes a delusion if we hold on to it and it lingers and remains as true for us after the experience is over.

These experiences as well as the thousand and one errors and misperceptions we make every day, the hundreds of thousands of miniscule or large misreadings we make throughout our lives, are not delusions unless we want them to be true and refuse to admit it to ourselves and others when evidence begins to mount more and more that they aren't.

Finally, we can be delusional, not only about inner and outer perceptions, but also about values. We can suddenly see someone as a terrible person only later to find they are no different from us. Or we may see someone as saintly or magnificent or a superman or superwoman only later to realize we must have been naive and delusional to have had such experiences. People who were degraded and nothing to us, suddenly become something, and the highest amongst us can suddenly appear low and corrupt.

In the way I'm using the word, there are delusions of the perceptual system, delusions of the belief system, and delusions of the system of values. And there are errors and hallucinations and illusions in all of these systems also.

And there are delusions we have personally, but a whole country can be delusional, and many think whole religions can inspire delusional states in all believing members. In the same way, one race may think of other races as either inferior or superior, and even though this may be an experience that is real and as plain as the nose on one's face, it can turn out to be delusion and false and unreal, and the awareness that one has, until this moment been delusion about other peoples, can come in a flash.

A few more points:

1) Simple errors can be pleasant or unpleasant. There are also pleasing delusions, for example, that we are loved by all or that we are bound to become rich and famous. In college I knew a man who thought all the girls were attracted to him and loved him. On the other hand, I knew people in college, quite nice and pleasant people, who believed ("knew") that no one liked them. Both poles were delusional, but one was a pleasant delusion and the other an unpleasant one.

2) I mentioned the obvious, that some delusions are quite dangerous to self and others, but some can be quite useful. For example, a man thinks he is fated to find the cure to pancreatic cancer. He works and works his whole career to find the cure, and never does, however he makes many important and useful and even life-saving discoveries along the way. If it hadn't been for the persistent delusion, he would never have made any of the discoveries.

3) Some illusions seem to stem from ones own personal, idiosyncratic perceptual apparatus and experience base. Some illusions, like the psychological illusions presented in every psychology class on perception, seem to stem from the human perceptual apparatus in general. Others are probably mammalian. (Some might speculate that even the most primitive animals and plants can have a type of hallucination and delusion.) Other illusions are peculiar to the individual.

4) It seems that delusions (unlike fleeting hallucinations or errors or mistakes or illusions) are need based. In these cases there seems to be something in us that needs to see things a certain way and that supports the way of looking at things and makes it persist in the face of all evidence. It is easy to see this in pleasant delusions — for example, where we think we are infallible or that we can not be seriously injured or that we can eat anything or do anything without it affecting us negatively — as we can see how they compensate our perceptions of our own weakness. It is more difficult to see why we might think we are awful people when we are not or that a spouse is unfaithful when he or she isn't (the Law of Compensation may help explain the process).

5) It is interesting, from a clinical point of view, that people who would never bow down to the dogmatic dicta of another living person (or of any author of a book), often accept, without question, the dogmatic dicta of some voice within that pronounces what is real and true. It can be useful for a person like this to think of the inner voice as just that, an independent inner voice or person, that should not be given into it blindly. Many marriages could probably be saved if the paranoid person would think to stand up to and argue with and cross examine the voice that pronounces their spouse;s infidelity. An inner dialogue would not remove the need (if there is such a need) on which the paranoia is based, but it would be a step in the process of self-knowledge that could eventually lead to the dissolution or containment of the delusion.

6) Illusions/Hallucinations/Delusions can be individual and isolated, discrete and self-contained, encapsulated and relatively separate from the rest of ones belief (perceptual) system, but they can also be giant chunks of a person's inner life, pretty nearly all encompassing. Some delusions can be like a tiny, but potent, alien germ that invades and infects a whole person. A person's paranoia can grow and grow, until the whole life is built around it. If the person doesn't withdraw totally in an angry, hurt state, he or she may still close off the feeling side and insulate the self from all intimacy and trust and, in this way, a sad and sorry state settles in on the person. This state can last a whole life time and become the person him or her self. It can become the person's personality and the person's reality.

7) If I understand the Buddhist idea at all, it seems Buddhists think that all life is a kind of illusion or need-based delusion. As we get older and older, as we reach a state where many of our needs have been met or are weaker and/or in which we realize they will never get met, life begins to feel more and more unreal with the things that seemed urgent and important fading into the dust of old memories.

It is interesting that the pleasant, naive, dream-like delusions of our youths, when they dissolve, can leave us frantic and miserable and desperate. But it is equally interesting and important that when the cynical and paranoid delusions of middle and old age begin to dissolve, there can be a feeling of a great liberation accompanied by feelings of joy and peace.