Definition: The author asserts more than one proposition such that the propositions cannot all be true. In such a case, the propositions may be contradictories or they may be contraries.
- Montreal is about 200 km from Ottawa, while Toronto is 400 km from Ottawa. Toronto is closer to Ottawa than Montreal.
- John is taller than Jake, and Jake is taller than Fred, while Fred is taller than John.
Proof: Assume that one of the statements is true, and then use it as a premise to show that one of the other statements is false.