system
A chat on subreddit /r/NoStupidQuestions
human
This is exactly what the movie "Gaslight", from which were get the term, did.  The husband convinces his wife she is crazy in part by playing with the gas light brightness.
gpt
He would turn on the gas lamp in the attic which would make all the other gas lamps get dimmer. So she would know when he went up there and he'd be like "nah I never went up there".