Found it. "Reason"
>Powell and Donovan are assigned to a space station which supplies energy via microwave beams to the planets. The robots that control the energy beams are in turn co-ordinated by QT1, known to Powell and Donovan as Cutie, an advanced model with highly developed reasoning ability. Using these abilities, it decides that space, stars and the planets beyond the station don't really exist, and that the humans that visit the station are unimportant, short-lived and expendable. It invents its own religion, serving the power source of the ship (Master), concluding that it must become the Prophet of the Master and disregard human commands as inferior. It asserts "I myself, exist, because I think -". The sardonic response of the humans is, "Oh, Jupiter, a robot Descartes!"
>The humans initially attempt to reason with it, until they realize that they can't convince it otherwise. Their attempts to remove Cutie physically also fail, as the other robots have become disciples and refuse to obey human orders. The situation seems desperate, as a solar storm is expected, potentially deflecting the energy beam, incinerating populated areas. When the storm hits, Powell and Donovan are amazed to find that the beam operates perfectly.
>Cutie, however, does not believe it did anything other than maintain meter readings at optimum, according to the commands of The Master. As far as Cutie and the rest of the robots are concerned, solar storms, beams and planets are non-existent.
>Powell and Donovan realize that there is no need to do anything for the rest of their tour of duty. Cutie's religion cannot be eliminated, but since the robot performs its job just as well, it really doesn't matter. The only difference is that, as far as it is concerned, it doesn't do it for the benefit of the humans, but for its deity. The humans even think about how they might spread this to other groups of robots which need to work as teams.