Gabriel Garcia Marquez

In journalism just one fact that is false prejudices the entire work. In contrast, in fiction one single fact that is true gives legitimacy to the entire work. That's the only difference, and it lies in the commitment of the writer. A novelist can do anything he wants so long as he makes people believe in it.

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.