Wayland's Principia

Alien: Unlike in appearance, history, and thought. Alien: Distant, too distant to reach, too far to travel in a universe bound by the speed of light. Alien: They did not come, but they sent word, words so strange that humans would have to devote their lives to mimicking alien appearance, taking in alien history, and following alien thought in order to understand them. No longer so human, generation by generation these Humans-Reaching-to-the-Alien become Guests on their own world.

Guests learning to think like five alien species so strange that they do not have names for names, let alone names for themselves.

Guests bringing gifts, the thoughts, ways of life, and technology of beings who share nothing in common with humanity or each other but the need for survival, the ability to think, and senses that look outward on the universe.

Earth grew used to these gifts delivered by alien voice and human hand, to alien medicine, alien power sources, alien social organization, alien mathematics. Earth grew comfortable with its Guests.

Until some of the aliens, on a ship in a long slow two-hundred-year arc from world to world, announce that they are coming to Earth to welcome their hosts and Guests.

Hearing of this, one voice cries out a different word for Alien:

Monster.