How do you choose what to believe? Or rather, how should you choose what to believe, in order to generally believe true things and not believe false things?

This is the most important question in the world!

For intensely abstract questions, about ethics or epistemology or math or the nature of the universe, the best strategy is sometimes “noodle it over,” but for most questions – how many people are coming to your book club, whether global warming is real, whether you left the oven on, what the magnitude of the gravitational constant is – you can’t conjure the answer from first principles.

So what do you do instead?

The specific strategy (e.g. check Wikipedia, look at the oven) depends on the problem. The completely general answer to all those problems is “search for some feature of the universe whose value correlates with what you want to know, observe the value of that feature, and infer the answer to your question.”

Some obvious examples, to help ground this idea:

• When you want to know how many people are coming to your book club, and you check the Facebook event to see how many people have RSVPed, you’re doing that because you (very reasonably) assume that the number Facebook presents has some relationship to the number of people who will show up.

• When you want to know if your coworker is a communist spy, you don’t just ask, because you know in your heart that a spy is approximately as likely to say “yes” as a non-spy.

• When you want to know if a coin is fair, flipping it 20 times and seeing how often it comes up heads is a good way to investigate, because “number of heads you get out of 20 flips” correlates strongly with large amounts of bias.

• When you want to know whether a coin is a penny or a nickel, flipping it 20 times and seeing how often it comes up heads is not a good way to investigate, because “number of heads you get out of 20 flips” doesn’t correlate strongly with penniness or nickelhood.

Some less-obvious examples, to show the usefulness of this concept:

• When you want to know what the most persuasive arguments are for the Demublican position on gay abortion, you shouldn’t ask that Repocrat-leaning site you like, “Why do Demublicans believe what they do about gay abortion?”, because which arguments you find will not be correlated with which arguments are actually strong1. (For the same reason, you also probably shouldn’t ask that Demublican-slanted site which, according your Repocrat site and Repocrat friends, Demublicans generally like and respect.)

• When you want to know if your coworker is a communist spy, and you peek in her purse and see that there isn’t a gun in there – well, absence of evidence is evidence of absence: if gun-presence correlates with spyness, then gun-absence correlates with non-spyness. It may be weak evidence, but it is evidence.

In addition, I claim this technique explains what’s wrong with every logical fallacy. Ad hominem: whether your opponent is a pedophile doesn’t correlate strongly with whether he’s right about special relativity. (Although whether he flunked out of college does.) Appeal to authority: whether Neil deGrasse Tyson supports the Raiders doesn’t correlate strongly with whether they’ll win the playoffs. (Whether your sports-buff friend does does.) Appeal to emotion: whether a picture of a starving child makes you feel sad doesn’t correlate strongly with how many starving children there are in the world. (It might correlate with whether starvation is “bad.”) And so on.

Anyway. I like abstract things that are so generally applicable that they’re almost useless. Here is one.

1. Unless you trust that the site’s devotion to the truth, even when it means presenting arguments that make its readers uncomfortable by shaking their faith in their own beliefs, will overcome the natual inclination to present weak arguments that make the readers feel good about themselves for being insightful and moral.