A single lie you tell yourself may seem plausible enough, when you don’t know any of the rules governing thoughts, or even that there are rules; and the choice seems as arbitrary as choosing a flavor of ice cream, as isolated as a pebble on the shore . . .
. . . but then someone calls you on your belief, using the rules of reasoning that they’ve learned. They say, “Where’s your evidence?”
And you say, “What? Why do I need evidence?”
So they say, “In general, beliefs require evidence.”
This argument, clearly, is a soldier fighting on the other side, which you must defeat. So you say: “I disagree! Not all beliefs require evidence. In particular, beliefs about dragons don’t require evidence. When it comes to dragons, you’re allowed to believe anything you like. So I don’t need evidence to believe there’s a dragon in my garage.”
And the one says, “Eh? You can’t just exclude dragons like that. There’s a reason for the rule that beliefs require evidence. To draw a correct map of the city, you have to walk through the streets and make lines on paper that correspond to what you see. That’s not an arbitrary legal requirement—if you sit in your living room and draw lines on the paper at random, the map’s going to be wrong. With extremely high probability. That’s as true of a map of a dragon as it is of anything.”
So now this, the explanation of why beliefs require evidence, is also an opposing soldier. So you say: “Wrong with extremely high probability? Then there’s still a chance, right? I don’t have to believe if it’s not absolutely certain.”
Or maybe you even begin to suspect, yourself, that “beliefs require evidence.” But this threatens a lie you hold precious; so you reject the dawn inside you, push the Sun back under the horizon.
Or you’ve previously heard the proverb “beliefs require evidence,” and it sounded wise enough, and you endorsed it in public. But it never quite occurred to you, until someone else brought it to your attention, that this proverb could apply to your belief that there’s a dragon in your garage. So you think fast and say, “The dragon is in a separate magisterium.”
Having false beliefs isn’t a good thing, but it doesn’t have to be permanently crippling—if, when you discover your mistake, you get over it. The dangerous thing is to have a false belief that you believe should be protected as a belief—a belief-in-belief, whether or not accompanied by actual belief.
A single Lie That Must Be Protected can block someone’s progress into advanced rationality. No, it’s not harmless fun.
Just as the world itself is more tangled by far than it appears on the surface, so too there are stricter rules of reasoning, constraining belief more strongly, than the untrained would suspect. The world is woven tightly, governed by general laws, and so are rational beliefs.
Think of what it would take to deny evolution or heliocentrism—all the connected truths and governing laws you wouldn’t be allowed to know. Then you can imagine how a single act of self-deception can block off the whole meta level of truth-seeking, once your mind begins to be threatened by seeing the connections.