The other day I wrote about Critical Thinking and how important it is. But knowing it's good for you and actually using it in your daily life are two very different things. I want to put together a Critical Thinking Toolkit.
One important tool is going to be Occam's Razor: "entities must not be multiplied beyond necessity" (entia non sunt multiplicanda praeter necessitatem). That's it in a nutshell right from William of Ockham, a Franciscan monk and English philosopher, theologian and logician in the 14th century.
Another way to put it is: The simplest explanation is usually the correct one. But don't get confused by the term, simple. It means: The hypothesis with the fewest assumptions is usually the correct one. When giving explanatory reasons for something, don't posit more than is necessary. Or, don't make any more assumptions than you have to.
So let's say you have 2 competing hypotheses that are basically equal in most respects. Then this principle would suggest that you choose the hypothesis that makes the fewest assumptions while still sufficiently answering the question. In science Occam's Razor is used as a rule of thumb (a heuristic) to help researchers develop good models.
In your life it can help you make decisions and choose what to think and what to believe (or not believe). You can use it as a heuristic as well, a great rule of thumb in your Critical Thinking Toolkit.
Sometimes atheists use Occam's Razor to argue against the existence of god since everything can be explained through natural means without complicating it with the supernatural.
Another example: Crop circles. There used to be 2 competing ideas for where crop circles came from. One was that flying saucers from an alien world made them. Another was that a person (or people) used some type of instrument to make the designs in the grass. Since there is no evidence for the flying saucers from outer space, and given how complicated and how many assumptions need to be made to make that argument work, Occam's Razor would suggest that the simpler explanation would be that humans did it with instruments. That is the argument that makes less assumptions.
Of course, the second argument could be wrong, but until there was more information, it was the preferable hypothesis. Then 2 guys admitted to the crop circle hoax in the 1990's. So that ended that debate for most people.
A quote by Carl Sagan is appropriate here: Extraordinary claims require extraordinary evidence. When it comes to the supernatural, Occam's Razor is a very valuable tool indeed.