Wikipedia describes Occam’s razor:
Occam’s razor (also spelled Ockham’s razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham. The principle states that the explanation of any phenomenon should make as few assumptions as possible, eliminating, or “shaving off”, those that make no difference in the observable predictions of the explanatory hypothesis or theory. The principle is often expressed in Latin as the lex parsimoniae (law of succinctness or parsimony): entia non sunt multiplicanda praeter necessitatem, which translates to: entities should not be multiplied beyond necessity.
This is often paraphrased as “All things being equal, the simplest solution tends to be the best one.”
In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest hypothetical entities. It is in this sense that Occam’s razor is usually understood.
This is supposed to be one of the basic philosophical principles of the skeptical movement. You know the skeptics, right? They’re the ones who say you shouldn’t believe in ghosts, psychics, and alien conspiracies—only I sort of hesitate to call them a movement because they haven’t made much progress in recent years.
It’s a good rule of thumb, though, as far as it goes. When you’re seeing unexplained lights in the sky, there’s no reason to suppose that they are alien spaceships. First, because that answer doesn’t really tell us anything about what the lights are and what they’re doing. And second, there are any number of simpler explanations for the appearance of lights in the sky, answers that don’t require you to assume the existence of something when there’s no evidence to support it.
Anyway, I have a corollary to add to the mix, called Smitty’s Razor.
“All things being equal, the most disappointing answer tends to be the correct one.”
That’s my new guideline, although I’ve been following it subconsciously for years. Use it any way you like, but don’t be surprised if the results are disappointing.