By Benjamin Kuipers
“No one has directly observed either electrons or fairies. Both of them are theoretical constructs, useful to explain observations that might be difficult to explain otherwise. The "theory of fairies" can actually explain more things than the "theory of electrons". So why do we believe in electrons, but not in fairies?
Is the issue a political one, where the "electron" fans got the upper hand in the nineteenth century, so by the twentieth century the "fairy" fans were a scorned and persecuted minority? Or, have we proved for sure that fairies don't exist?
No, to both. The real difference is that for electrons, we have accumulated a set of quite narrow and specific rules about how electrons will behave under various circumstances. Those rules let us make very specific predictions about electron behavior, and about the observations that will result. If those predictions don't come true, we know that either we didn't set up the circumstances correctly, or there is something wrong with the rules. But over many decades, we have repeatedly fixed problems with the rules, so we can now make really good predictions about electrons, especially in certain highly contrived circumstances (i.e., circuits).
Fairies are much more free. A fairy does what it decides to do. We haven't been able to find any useful rules for predicting how a fairy will behave under particular circumstances, or even for telling when a fairy has been involved in a particular observation. (At least I don't know of any such rules. I stand ready for correction on this.) Over many, many decades, it has not been possible for people to try out pretty-good sets of fairy-prediction rules, find out where they make mistakes, and replace them with better sets of rules.”