Beliefs are tools, not trophies.
Hold an idea with the conviction its evidence earns, and let it go without embarrassment when better evidence turns up.
Pluto is the clean example. When astronomers demoted it in 2006, people who had not thought about Pluto since a grade-school poster were suddenly upset. Nothing about Pluto had changed. What changed was a definition, because the people whose job it is to define planets had found other objects out past Neptune that looked a lot like it and needed a rule that covered all of them.
The upset had little to do with astronomy. A fact learned in childhood feels like a possession, and a committee had just taken one away. That is what it feels like when a belief has turned into a trophy.
A tool is different. You keep a tool as long as it works and replace it when something works better, and nobody is ashamed of replacing a tool. The idea here is to hold beliefs the same way. If the evidence for something is strong, hold it firmly. If it is weak, hold it loosely. If it changes, change with it, and skip the part where you feel stupid for having believed the old thing. You were not stupid. You were using the best tool available at the time.
Childhood is where most of the trophies come from. Things learned early get filed as simply true, because at seven you had no category for "the current best understanding." So the exercise is to go back through the shelf now and then and ask of each item: do I believe this because the evidence holds up, or because it was there when I was small? Pluto can be whatever the astronomers say it is, and you have lost nothing.
III. Intelligence can be practiced.