The function of definitions is to make clear and unambiguous the meanings of the terms we use. When precisely formulated and conscientiously followed, definitions in turn make our reasoning more exact and our communication more successful. Unless its meaning is clearly understood, no statement can be empirically verified nor logically validated by reference to any other statement. In all scientific fields, but particularly in psychiatry, such clarity of meaning can be achieved only through the most scrupulous efforts to specify these meanings in carefully formulated definitions. If such definitions are to satisfy the pragmatic criterion of "working successfully," the descriptive terms in them must, wherever possible, refer to public matters of fact that can be established by operational procedures that are feasible to carry out by suitably trained investigators. Furthermore, all implicit inferences to causal antecedents or to future consequences, to subjective processes that can be directly experienced by only one subject or to unconscious factors (in the dynamic Freudian sense) that can be directly experienced by no subject, but merely inferred—all such more or less dubious inferences must be brought out into the open and clearly so labeled, and not, as so often happens, be left concealed under the verbal cloak of the definition as formulated, where they create endless confusions. Definitions so conceived, so formulated, and so used do not limit our intellectual freedom to say anything we mean, but are, rather, the logical instruments by which alone what we mean can be said, and this intellectual freedom become, accordingly, a positive condition of self-expression and successful communication.