Carroll Quigley

Islam, the third in historical sequence of the ethical monotheistic religions of the Near East, was very successful in establishing its monotheism, but had only very moderate success in spreading its version of Jewish and Christian ethics to the Arabs.

The argument that the two parties should represent opposed ideals and policies, one, perhaps, of the Right and the other of the Left, is a foolish idea acceptable only to doctrinaire and academic thinkers.

The historical novelist has to consider what has actually happened, while the SF writer is dealing in possibilities, but they are both in the business of imagining a world unlike our own and yet connected to it.