We see with the eyes, but we see with the brain as well. And seeing with the brain is often called imagination.
Every act of perception, is to some degree an act of creation, and every act of memory is to some degree an act of imagination.
With any hallucinations, if you can do functional brain imagery while theyre going on, you will find that the parts of the brain usually involved in seeing or hearing - in perception - have become super active by themselves. And this is an autonomous activity; this does not happen with imagination.
