Before thinking about hallucination we should understand what is our sensory Mechanism. We have five basically five senses hearing, sight, Vision, taste and touch. The role of sensory organs is just to collect impulses. They don't know about what impulses are they. They transfer it to the brain for processing. For instance the eyes were capture the picture and gives to brain and the brain process it and tells that it is the picture of X. Then we experiencing that picture. The brain is using neurons and neuro transmitters are the communication mechanism which the brain is using. If the neuro transmitters are bad of improper brain will get false messages for processing. And we feel experience some thing we have seen which is not actually there. So hallucinations are purely false sensory perception of brain.
