Occam's Razor

One of the best tools to have available to you is Occam's Razor. One way to define the function of OR is:

"When competing hypotheses are equal in other respects, the principle recommends selection of the hypothesis that introduces the fewest assumptions and postulates the fewest entities while still sufficiently answering the question." 

In English that means: if you have two or more equally solid ideas about how to explain something, the idea that is the most simplistic is the one to choose. Of course this doesn't mean that a really simple -- but completely invalid -- theory should be chosen over a valid one that is more complex; it's just a "rule of thumb" that instructs us to keep it simple.

One example of OR in action is its use in investigating religious experiences. Say we have a test subject named "Greg", who is a believing and practicing Christian, and he claims to hear the voice of a God in his head. Let's suppose we hook Greg up to an MRI machine, and we measure and record his brain activity while he is having a religious experience; we study this until we have a solid understanding of the effect of revelation on the brain. Now, if we hook an Atheist up to that MRI machine, and we observe the exact same changes in his brain while we simulate a religious experience, we can thus eliminate god from the picture via Occam's Razor; since we were able to replicate the effects of revelation without a God being involved, it's simpler to just say that the God isn't necessary for these brain activities to occur.

Another example of OR doing its thing would be crop circles. There were two leading ideas about how these circles happened, some say aliens did it, while others argued that humans were responsible. Since the idea of aliens being the cause is a highly complex one, we can eliminate it in favor of the more simplistic explanation that humans were behind the phenomena -- which turned out to be right. 

I would even argue that OR can help us with the Evolution vs Creationism debate. Both ideas are complex, but Creationism is far more complex than Evolution, and thus we should prefer the more streamlined explanation that we came to our current form by changing over time.

Occam's Razor is still sharp even after 700 years of use. Thanks William!