Consider the proposition p
Any statement
For example,
The sky is blue
or
Pittsburgh is cold today
What does it mean to doubt p
I might test my doubt by my behavior
I doubt that the door is locked
I doubt that the door is locked, so I go back and recheck it
this sounds a little neurotic
Or perhaps it is a kind of game
Could I doubt right now that I am in class?
Maybe I try to doubt and fail
Could I doubt that I am in Pittsburgh?
I travel, and maybe when I wake up I forget what city I am in
So I could forget I have returned home- that is possible
Could I doubt that I am on the earth?
This sounds like a game.
Like a science fiction movie