The remarkable skill with which children intuitiviely use spoken language demonstrates that knowledge of semantics is not a necessary condition for fluently speaking a language. However, the sensible and responsible use of language in a science such as medicine requires more than intuitive linguistic behavior. The application of terms, especially of disease terms to patients in diagnoses such as "Elroy Fox has angina pectoris", often has many serious consequences. In light of this, one ought to be well acquainted with the syntax and semantics of one's terms, so that one may choose the " right words' in communicating one's observations and experiences. Maybe Elroy Fox does not have angina pectoris, but pneumonia? How will one differentiate between a situation where one term is appropriate and the other not, and another where the reverse is true? Technical problems of this type amount to asking the semantic question: "What does the term such-and-such mean? ", or equivalently, "how is the term such-and-such defined in medicine? ".