The law of non-contradiction is a rule of logic. It states that if something is true, then the opposite of it is false. For example, if an animal is a cat, the same animal cannot be not a cat. Or, stated in logic, if +p, then not -p, +p cannot be -p at the same time and in the same sense. The law was stated as a principle of mathematical logic by Russell and Whitehead in "Principia Mathematica". 