Tom Stoppard

When I was younger, I could do something useful just by being free for half a day, but now I need five days to get the world I've left out of my head and ten days or a fortnight not talking to anyone to hold what I need to hold inside my head.

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.