If thoughts had to be spoken before you could believe them, embarrassment would become part of reasoning.

Having to say things aloud before believing them would put embarrassment inside the process of thinking. A great many beliefs survive only because they are never exposed to the sound of themselves, and speaking them would either kill them off or force them to be improved. Privacy protects bad reasoning.

You can approximate the test cheaply. Say the belief out loud to a person, or write it in a sentence you would be willing for them to read. Anything you would be uncomfortable stating plainly is worth examining, not because embarrassment means wrong, but because it usually marks a place you have avoided looking at.