You're in the middle of a long hike through the jungle when you're bitten by a venomous snake. You run to the nearest building you can find, which, lucky for you, happens to be the local witch doctor's. Unfortunately, the witch doctor is out. His six assistants--Abe, Beth, Cory, Dan, Ethan and Fred--are there, however. It is known that two of them are knights, two are knaves and two are liars.
You see that there are six differently colored potions sitting a shelf marked "snake-bite antidotes"--red, yellow, green, black, purple and white--and you describe the snake that bit you and ask which potion you should take. The following conversation ensues:
Fred: "Oh, of course--it's the black one or the white one."
Abe: "It's either black, yellow or purple, or Fred is a liar...but not both."
Ethan: "No, it's either red or purple, not green."
Dan: "Ethan, you're a liar."
Ethan: "No, I'm not, but you are."
Dan: "OK, well, if I'm a liar, then you're right, it's red or purple."
Cory: "You are a liar--I'm not, though."
Abe: "Yeah, Cory or Fred is a knight."
Fred: "Personally, I'm a knave, and it's either the green, white or purple."
Beth: "No, it's either the yellow, green or red."
Ethan: "No, it's none of those."
Cory: "It's yellow, white or red, not green."
Beth: "I guess you should listen to Cory--he's a knight."
Who's what? And which potion should you take?