Distinguishing Reality, Illusion, and Delusion

We live our lives in a huge illusion, but we presume there is a reality "out there" which generates the illusion we experience.

For example, the sky is not blue.  The atmosphere scatters certain wavelengths of sunlight more than others, with the net result that what enters our eyes when we look skyward are wavelengths that causes our brain to generate a certain experience that our mind in English calls "blue."  In other words, colors are an illusion of the brain generated by the fact that when we look at things, whatever wavelengths of light they reflect (or in a few cases generate) are made by the brain into experiences (or color "qualia") for our mind.

I sometimes hear voices that aren't there (usually when half-asleep) that disturb me, but I long ago realized that if they are not there, then they are not there and it is something generated in the brain, no matter how "real" they sound.  Sounds are not real.  The brain generates sound qualia just as it generates color qualia, and in both cases, although they usually have some relationship to the reality of the external world, they don't always.

The difference, of course, is that the sounds of Bach in the background are illusions while voices I may hear from phantoms are delusions.  The Bach comes from sound waves from the speakers in my room from the CD I'm playing (an illusion generated by another illusion generated by another) while voices in the head no doubt have some source in my normal mental chatter that gets mistaken as incoming sound, but it has no external source and so is not illusion but delusion.

Illusions have something "real" under them, delusions do not.