Motte and Bailey is a concept by Nicholas Shackel that Scott Alexander has popularised. He describes this as follows:
The original Shackel paper is intended as a critique of post-modernism. Post-modernists sometimes say things like “reality is socially constructed”, and there’s an uncontroversially correct meaning there. We don’t experience the world directly, but through the categories and prejudices implicit to our society; for example, I might view a certain shade of bluish-green as blue, and someone raised in a different culture might view it as green. Okay.
Then post-modernists go on to say that if someone in a different culture thinks that the sun is light glinting off the horns of the Sky Ox, that’s just as real as our own culture’s theory that the sun is a mass of incandescent gas. If you challenge them, they’ll say that you’re denying reality is socially constructed, which means you’re clearly very naive and think you have perfect objectivity and the senses perceive reality directly.
The writers of the paper compare this to a form of medieval castle, where there would be a field of desirable and economically productive land called a bailey, and a big ugly tower in the middle called the motte. If you were a medieval lord, you would do most of your economic activity in the bailey and get rich. If an enemy approached, you would retreat to the motte and rain down arrows on the enemy until they gave up and went away. Then you would go back to the bailey, which is the place you wanted to be all along.
So the motte-and-bailey doctrine is when you make a bold, controversial statement. Then when somebody challenges you, you retreat to an obvious, uncontroversial statement, and say that was what you meant all along, so you’re clearly right and they’re silly for challenging you. Then when the argument is over you go back to making the bold, controversial statement.
Sometimes Motte and Baileys arguments are the result of bad faith, but I suspect in many cases that those making them have no idea that they are engaging in such a strategy. In fact, it seems highly likely that one or more Motte and Baileys would be popular among rationalists. What are the most common such Motte and Baileys?