Yeah, the tricky part about probability is that it depends on counterfactuals - things that could have happened but didn’t, based on your knowledge of the other person’s behavior or the world’s behavior, still affect the probability.
If that seems crazy in the abstract, it’s because human brains are bad at abstract math. And they’re bad at reasoning about cases that don’t happen in real life. But there are a lot of specific real-life cases where it’s pretty intuitive, actually. Suppose a particular friend doesn’t call you today…
- If you weren’t expecting a call, then that’s business as usual, nothing surprising.
- If normally they call and you chat daily and they basically never skip, then them skipping might lead you to think that they’re unusually busy and or something is wrong.
So the same literal observation (no call) gives you a very different conclusion based on your knowledge of the other person’s behavior.
So in the case of Monty Hall, it turns out:
A In the standard version, if Monty always opens one door that doesn’t have the car (taking advantage of his knowledge of where the car is), then switching is better, 2/3 to 1/3.
B If Monty opens doors randomly, or forgets where the door with the car is, and takes a guess himself and just happens not to reveal the car in a particular case, then switching doesn’t help, it’s 1/2 to 1/2.
C If Monty is out to fool the people who have memorized the standard answer and only ever offers the option to switch in the first place if you initially pick the correct door (if you initially pick the wrong door then he just makes you open it and that’s the end of the game), then in the situation where you’re offered to switch, switching is bad, it’s 0 to 1.
In all three cases, A,B,C, you might in a particular run end up facing the same thing - being offered to switch after seeing a door with a goat opened, but the probabilities are all different because, like in the simple phone call case, what you infer from a given observation changes based on what you know about the other person’s behavior. In case C, obviously if you’re facing a choice to switch, and you know with certainty Monty behaves this way, then you definitely shouldn’t switch.