A science of society is possible - but its application is distorted by social conflicts.
A cyclical theory of history applies to future civilizations.
A police force is necessary to prevent time travelers from changing history.
Corridors constructed in space can be rotated onto the temporal axis.
Time travel is a psychic power.
A few mutant immortals survive through history into an indefinite future.
A relativistic spaceship accelerates indefinitely.
Faster than light travel is discovered and spreads but there are so many intelligent races that it is possible to communicate and trade only within a local "civilization cluster" and some human interstellar explorers return to the Solar System to find that Earth has been sterilized in their absence.
Lifespans are extended indefinitely, instantaneous intergalactic travel is possible and an intergalactic planetary system is inhabited.
Inorganic intelligences spread through a mostly lifeless galaxy at sublight speeds.
Human consciousness can be preserved in artificial cybernetic systems.
Human and animal intelligence suddenly increases.
Magic is a set of technologies. (From Heinlein.)
Shakespeare's plays were histories.
Medieval Englishmen do not know that they cannot defeat aliens with interstellar capability...
At sublight speeds, interstellar traders live in their ships.
After a nuclear war, civilization is rebuilt in the Southern Hemisphere.
Teleportation sends fuel to interstellar ships and is used to travel to extrasolar planets that the ships have reached.
Nantucket time travels.
Technology stops working.
When a comet strikes Earth, the British Empire relocates to India.
Slave-owning society survives and thrives into the twentieth century and beyond.
Mars and Venus were terraformed a long time ago.
A small group from an Earth like ours colonizes a North America that had never been invaded by Europeans.
Premises And Conclusions
A conclusion can contradict its premise. The purpose of Anderson's temporal police force, the Time Patrol, turns out to be to counteract temporal chaos.