 It is based on the deeply rooted intuition that "revealing information that is already known does not affect probabilities".But, knowing that the host can open one of the two unchosen doors to show a goat does not mean that opening a specific door would not affect the probability that the car is behind the initially chosen door.The point is, though we know in advance that the host will open a door and reveal a goat, we do not know "which" door he will open.