Story, first published: Galaxy Science Fiction, Sep.1957
An expedition is preparing an Earth-type planet for a colony. To prove they are adult members of the galactic community, the humans have come to the planet to start over, and not waste its natural resources, not "gut this world like man did Earth".
The expedition is falling behind schedule because their machines are constantly breaking down. The workers blame the Shadows, alien beings who have attached themselves to each man in the camp. The Shadows do not attempt to communicate with the humans, but merely hang around observing and are found to be making tiny models of everything in camp, including the humans. When one of the Shadows looks into a peeper, a forbidden device that can create fantasies dangerous enough to trap men's minds, the Shadow vanishes. Desperate because an inspector is coming to check up on their progress, the expedition tries to trap all the Shadows using the peeper. The humans conclude that the Shadows are alien proxies, machines to keep track of human activities. Another alien appears with life-size duplicates of everything in camp. He will deliver as many machines as they want, as long as the humans provide entertainment with the peeper. It does not look like the aliens are getting the better of the trade in "technologies," but then products of human imagination seem to have a high value in the galactic market.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.64-65