I'll definetly look up the theory of solipsism and prince andrei's inner monologue.. sounds really interesting and am sure a lot of thinkers came to that conclusion during a day...
One interesting argument would be a research I read about.. where people got arrested lonely in prison's for example they started to create another persons. Just as imagination or maybe as a drawing into the wall. They began to talk or even to argue and have a fight with them.. It's hardly to say if this "experiment" agrees or disagrees with solipsism..