Count Dooku : What if I told you that the Republic was now under the control of a dark lord of the Sith?
Obi-Wan : No, that's not possible. The Jedi would sense it.
Count Dooku : The Dark Side has clouded their vision. Hundreds of senators are now under the influence of a Sith lord called Darth Sidious.
Obi-Wan : I don't believe you.
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From: Star Wars: Episode II - Attack of the Clones (2002)