Rational Beliefs Must Be Able to Be Tested

Suppose a person stops you on the street and tells you that the world started to exist just five minutes ago.

Puzzled, you tell them you have all kinds of memories about how the world has existed for longer than five minutes.

They respond by telling you that the memories were implanted by a divine trickster who created everything five minutes ago and likes to make people think they have existed longer than they really have.

You ask the person if they have any reason to think the divine trickster really exists, and they respond by telling you that if the divine trickster gave clues about his existence and intentions, then they would not be a very good trickster after all.

Flustered, you realize that no matter what evidence you raise, your annoying acquaintance will just explain it away. Everything about their crazy theory is coherent. You cannot find any contradictions. Even though they sound insane, you cannot put your finger on anything wrong with their argument because they can make it pass any test you can throw at it by inventing additional new beliefs that explain anything away.

That is exactly the problem, though. In order to counter all of your objections, the crazy person makes it so their story could never be deemed false by tacking on new beliefs.

In order to be rational, we want to be able to think of ways we could tell if our own beliefs are false. Think of it this way: every time we can think of a way our belief could be proven false through a test but our belief passes the test, we gain reason to think our belief is true.

If we need to keep adding on new beliefs just to save another one of our beliefs, then we very likely have a serious problem. Sometimes we do need to add on new beliefs or change some of our old ones in order to accommodate a belief that is still worth having (for example, astronomers in the past changed their beliefs about the number of planets in the solar system to accommodate their astronomical model – and it turns out the planet really was there!), but the less reason you have for your new beliefs, and the more of them you have to invent, the worse off you are.

Looking for ways a belief could be proven false is how we avoid conspiracy theories. For example, we could come up with explanations for why we should think the world is flat despite the evidence to the contrary, but we would need to make too many additional beliefs to make a flat earth fit our experience.

The same is true of our political and religious beliefs. If there is no way someone could ever give us evidence that our religious and political beliefs are wrong, then we probably have the wrong beliefs to begin with. If our beliefs can pass tests without adding on more beliefs, then we have reason to think our beliefs are good ones. But if we need to keep adding more beliefs, then there is probably a serious issue.

Consider the belief that Donald Trump is a perfect President and anybody who says otherwise is in on an evil Deep State conspiracy theory. There are no tests this belief could ever pass because the person who believes it has invented additional beliefs that make it immune to any and all tests. Any time someone gives evidence to the contrary, the person holding this belief just lumps them in with all the people who must be in on the conspiracy.

If a belief can pass every test, then it does not really pass any tests.

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