Doublethink is a very wierd and confusing concept. It is when you know something to be true one minute, and the next minute know it to be totally opposite of what it was. When Obrien is questioning Winston it was very confusing and interesting the think about at the same time. Everytime Winston knew he was right, Obrien just explained it away with doublethink or talking about the party altering the past. Winston ended up very confused on how Obrien could be right and how it was poosible.
I do not think doublethink is possible because someone would just have to be tricking themselves that something is true, but there will always be doubts in the back of his mind because he knows the real truth. The party cannot destroy the power of human reason and inteligence so there will always be people that do not believe in doublethink or believe what the party says. I am just at the point in the book when obrien is trying to convince winston that even though he is holding up 4 fingers it really is 5 because obrien says it is. I think that there is no way on earth that a person can decieve themselves that much that they would believe totally 100% that there were 5 fingers being held up.