Formal definitions specify what is necessary and sufficient for the identification of a particular term. These formal definitions use precise language and do not admit contradictions; they are exact class. There are multiple advantages of exact class definitions. They enable us to confidently use deductive arguments so we can ensure that the terms in the premises match the terms in the conclusion. They prevent sloppiness and circularity of logic. They also help us look beyond common sense or what we think we already know. I illustrate these advantages and also the problems that can occur when definitions are not exact class.