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