I'm currently ready this book called "The Demon-Haunted World" by Carl Sagan, and in it was a brief chapter on Hallucination. The book talks about hallucination as a reason people might believe in things that aren't real, like abductions or ghosts (or God). Well I found it fascinating and thought it would be a fun topic for the blag.
First let's do some background. A hallucination is generally defined as perceiving something without a stimulus. Now hallucinations aren't limited to vision. You could definitely hear voices where there are none, or smell pizza when there's none around. There are of course some treatable forms of reoccurring hallucinations, but it's not uncommon for regular people to experience them. They are especially likely right before going to sleep or right after walking up. There are also drugs that can make you experience hallucinations, like LSD for example. Sleep and food deprivation, along with isolation, can increases the likelihood that you will experience hallucinations. The Native American vision quests are a good example of this.
Dreams can be considered a form of hallucination. There is a particular state during the sleep cycle when dreams are believed to occur. The state is called REM, or rapid eye movement. As you might guess, your eyes move erratically and your brain activity increases. According to Sagan, there was a study in which people were awoken during the night every time they entered the REM state. After about a week, these people began experiencing hallucinations during the day. So it would seem that we hallucinate naturally, but normally during the REM state. We might also observe that removing the REM state will increase daytime hallucinations. Of course this is just theory, but it's an interesting thing to consider.
So there you have it. A perfectly good explanation for the extraordinary. Some people want so much for the world to be something more. I can't understand why. Reality already offers so much to be fascinated in.