James Stephens

There is no tragedy more woeful than the victory of hate, nor any attainment so hopelessly barren as the sterility of that achievement; for hate is finality, and finality is the greatest evil which can happen in a world of movement.

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.