He will permit any speculation at all — as long as it is about gadgets only and doesn’t touch people.He doesn’t care what mayhem you commit on physics, astronomy, or chemistry with your gadgets… but the people must be the same plain old wonderful jerks that live in his Home Town. Give him a good ole adventure story any time, with lots of Gee-Whiz in it and space ships blasting off and maybe the Good Guys (in white space ships) chasing the Bad Guys (in black space ships) but, brother, don’t you say anything about the Methodist Church, or the Flag, or incest, or homosexuality, or teleology, or theology, or the sacredness of marriage, or anything philosophical! Because you are just an entertainer, see? That sort of Heavy Thinking is reserved for C. P. Snow or Graham Greene. You are a pulp writer, Bud, and you will always be a pulp writer even though your trivia is now bound in boards and sells for just as much as Grace Metalious’ stories… and you are not permitted to have Heavy Thoughts. Space Ships and Heavy Thinking do not mix — so shut up and sit down!–Robert A. Heinlein
The rule is: Science Fiction by its nature must be trivial.
This of course rules out… a large fraction of my work — and all my future work, I think
Thanx to andrewducker