Gill,
You are basically discussing the nature of perception. Perception is all that we have to interpret the world around us. We know that perception can be altered by mental illness and drugs (famously, LSD and the related ergot – a fungus on wheat
[1].
Have a look at
http://tlc.howstuffworks.com/family/brain-religion.htm and see the comments on St Paul. His symptoms – flash of light, falling down and hearing voices, are now instantly recognisable, not as a miracle but as temporal lobe epilepsy that distorted his reality, probably in line with guilt feeling about persecuting people. In the cases of drugs and mental illness, the qualia that a person experiences are erroneous; men from Mars are not beaming radio waves at them, the voices they hear do not exist and the subject does not have the ability to fly, no matter what their brain tells them.
Then there is the second issue with qualia; misinterpretation. Our brain, which controls the qualia, is designed to find reasons for, and patterns in, the events that qualia reveal to us. If we do not know the answer, the brain will come up with its "best guess". We have a tendency to trust some people when they give explanations, although those explanations might be wrong. Also, we like to give things names so we can discuss them with others.
As an example, I suffer from long-sightedness. It is not uncommon but one effect is that, in dark conditions, the pupils open up, to gather light. This means that I do not focus well on things at a distance, although nearer and brighter things are seen perfectly. I discovered I was long-sighted when driving at night. Out of the corner of my eye, I would see shadowy people in bushes, posts that would move, etc. Were these ghosts? “No.” Once I had a pair of glasses, there was no longer a problem. But 2000 years ago, there were no glasses, but people still had long- or short-sightedness, and interpreted what they saw as ghosts, demons, spirits, etc. Others had similar experiences and this misinterpretation of qualia provided an explanation. We know better now, but some people stick with the wrong idea. Once they start to believe erroneously, they have a personal difficulty rejecting their idea. This is known as cognitive dissonance. They know it is wrong, but will not admit it to themselves.
http://en.wikipedia.org/wiki/Cognitive_dissonanceFinally, think of a blue hippopotamus.. OK? Now think of an invisible hippo... OK? We can think of [anything + any attribute.] You will agree that just because we have an image of something in our mind, does not mean that it exists or has ever existed.
I hope now, that you understand how erroneous qualia and the erroneous interpretation of qualia leads to erroneous conclusions.
At the end of the day, it does not matter if qualia are measurable in units, they are measurable against normality.
