"Knowing everything" implies far more than our intuitions may initially tell us, and can lead to "a set of all sets" type of paradoxical problems (i.e. the premise of the argument is absurd). This and other philosophical thought experiments like the Chinese room abuse our intuitions about being human, learning things, performing tasks, etc.
In this particular experiment, if Mary's omniscience allows her to invoke the specific pattern of neural activation that is triggered by observing a blue sky, then she won't learn anything new, otherwise she will. It all depends how far we take the absurd claim of her omniscience.
In this particular experiment, if Mary's omniscience allows her to invoke the specific pattern of neural activation that is triggered by observing a blue sky, then she won't learn anything new, otherwise she will. It all depends how far we take the absurd claim of her omniscience.