Validity An argument's property in logic is that the validity of the premises logically assures the truth of the conclusion. Because of the nature of the argument, whenever the premises are true, the conclusion must likewise be true. A premise is said to be valid if it cannot possibly be false; otherwise, it is invalid. In formal arguments, the word "validity" also refers to the property of a logical system or framework within which the argument is written down. If a formal system has this property, then we say that the argument is "valid" in the system. For example, Peano Arithmetic is a formal system whose axioms guarantee that any proof can be reduced to an empty one. Thus, any argument in PA is valid. In informal arguments, the word "validity" often has a different meaning. Here, we say that an idea is valid if it is not possible for it to be false; that is, it must be true. For example, let's say that someone claims that all swans are white. We could ask them to prove this statement by giving us one black swan to disprove their claim. They could not do so because it is impossible for a black swan to exist - therefore, their claim was valid. Now, suppose they had claimed instead that almost all swans are white.