Ernest Hemingway

But if, on the other hand, we should be justified in rejecting it, if there testified on oath, then, supposing our rules of evidence to be sound, we may be excused if we hesitate elsewhere to give it credence.

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.