How do you tell reality from fantasy?

I asked a few people that question the other day. Mostly they rolled their eyes and indicated that they thought it was a retarded question. "You just do", was the most common response.

I thought this was a very interesting response given that the people to whom I posed the question were a group of atheists and theists trying to "convert" one another. How can you hope to have an impact upon any thinking person if you cannot explain how you arrived at your conclusions? It really didn't seem to matter; the theists had been reduced to repeating variations of Pascal's Wager while the atheists were denigrating the theists' intelligence (the phrase "silly poopyhead" was actually used by one of the "intelligent" atheists...irony is sweet).

Telling reality from fantasy seems like one of the more significant things a human being does and deserving of a better explanation than "just because". I mean, if you don't understand how to differentiate reality and fantasy, how do you know that you are?

Schizophrenics have a hard time with this, at least from the point of view of a "normal" person. If a normal person hears a schizophrenic assert that Rigellian Slobber-Monkeys have taken control of Barack Obama's brain, they will merely smirk and dismiss the allegation as patently false. The schizophrenic himself would contend that his perception of reality is true and everyone else is either blind to the situation or they are Rigellian Slobber-Monkeys themselves.

Our perceptions of reality are derived from input from the five senses that we all learned about in elementary school - sight, hearing, touch, taste, and smell. The brain integrates and interprets sensory input and tries to make sense of it. The potential for error is enormous.

None of our senses is perfect. Our eyes can only see limited wavelengths. Our ears only register sounds within a narrow range of frequencies. A light enough touch, a dilute enough flavor, or a faint enough smell will go completely unnoticed by the associated organs. The information getting to our brains is pitifully imperfect.

What happens next can vary from the ridiculous to the sublime. Under the right circumstances, the human mind can take this imperfect information and do remarkable things with it...cure smallpox, prepare a five-star meal, etc. But in an unsettlingly high number of instances, the brain screws it all up.

Eye-witnesses to a crime often give conflicting description of both perpetrators and events. Studies have been done that indicated this can be due to lighting and distance from the scene, but they also indicate that this can often be attributed to imperfections in how the brain processes information.

It becomes apparent that we cannot rely just on our senses and what we think we saw/heard/felt/tasted/smelled. We need to find a way to verify our experiences.

This where science comes in.

Science offers the closest thing to an objective evaluation of reality that the human mind can achieve. Through a process of testing and experimentation, we can discern what is real from what we merely wish was real. This methodology is not reserved to owlish academics with endless initials after their names; everyone can apply it to their lives. Why go to all the trouble? Because beliefs rooted in a perception of reality that has been objectively evaluated are more objectively true than beliefs rooted in myth, superstition and/or wishful thinking (aka "faith").

Unless you're looking for a justification for bombing an abortion clinic or setting yourself on fire.