Reading and writing about things that may be possible requires imagination. Sci fi is all about extrapolating the known to extend into the possible. Any person has a window of reality that they can peer through to see the things that are. With an expanded imagination a person has an infinity of windows through which they can see the things possible. You explore worlds unknown. You explore worlds unimagined. You are not imprisoned by reality.
(Thanks to Richard Lee Anderson for his definition)