"Only a Sith deals in absolutes" is a famous quote from the 2005 film Star Wars Episode III: Revenge of the Sith. It is often quoted as a tongue-in-cheek axiom to imply that any statement expressing certainty or fact is that of an evil Sith.

In the film, Anakin Skywalker, now known as Darth Vader, (played by Hayden Christensen) has turned evil, and confronts his wife Padmé (Natalie Portman) and his mentor Obi-Wan (Ewan McGregor). Anakin tries to convince Obi-Wan of his cause, but his language convinces Obi-Wan that he has become a evil Sith.

Anakin: "Don't make me kill you."
Obi-Wan: "Anakin, my allegience is to the Republic, to democracy!"
Anakin: "If you're not with me, then you're my enemy!"
Obi-Wan: "Only a Sith deals in absolutes."