Thomas J. Watson

Nothing is beautiful, only man: on this piece of naivete rests all aesthetics, it is the first truth of aesthetics. Let us immediately add its second: nothing is ugly but degenerate man - the domain of aesthetic judgment is therewith defined.

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.