Emerson said of the left and right, "Each is a good half, but an impossible whole." In public affairs, the left traditionally pushes for political, economic and social progress, while the right steers towards individual opportunity, stability, tradition and the maintenance of gains already made. Their disagreement is the basis of most of our political discourse. Drawing on four decades of field research on partisan conflicts in the U.S., Northern Ireland and Israel, the author provides a clear, concise explanation of how the left and right think and why their respective philosophies reflect competing views of human nature.