In chess, pawns represent the infantry. They are the expendable ones the insignificant foot soldier. Pawns, however, can eventually be converted into higher pieces, most commonly queens, the most powerful piece on the board. Porridge is a king who aspires to be a pawn. He has thrown off the cloak of a horrific responsibility and hidden himself away on a backwater world. Like Alec Palmer in Hide, hes very aware of his role in sacrificing lives to a terrible war. This parallels the Doctors role in sacrificing Gallifrey in the Time War. Both are charged with protecting the universe at great personal cost to themselves. Clara appears to be a pawn at first glance, but is raised up by the Doctor to a leadership position. She has no trouble assuming the role. In every episode Clara appears in she changes to fit the situation. Has she been engineered somehow to be the Doctors perfect companion? She is reminiscent of the TARDIS in the sense that she becomes whatever the Doctor needs her to be. Was this another test for Clara? Is the Doctor trying to see how far he can push her? Remaining a pawn is contingent upon a belief in your place in the hierarchy. You are powerless because you have been told that you are. There is no inherent superiority to a king or queen; its simply luck and timing. The cybermen are representative of what happens when a desire to level the playing field results in a loss of individuality and creativity. The cyber-Doctor is defeated because he cant think outside of his data network.