Possibility Generator

The Possibility Generator was a creation of the Doctor which allowed him to explore virtual worlds and ideas. Susan said it was illegal to have a possibility generator. It was apparently a chamber that the Doctor could sit inside, allowing information to be projected into his mind as he played out scenarios, but he could include others in the illusion if he wanted, creating more elaborate virtual realities. (AUDIO: Auld Mortality)

Later the Doctor used the possibility generator to create an illusion of Susan so he would have her with him on his travels. When the Time Lords caught up with for changing history the illusion of Susan was breaking down as the probability generator ran out of power, the Doctor claiming to Susan that she was just ill in an attempt to hide her true nature from her. The real Susan took her place while the illusion was returned to Gallifrey to resume the role of President and die. (AUDIO: A Storm of Angels)