4.5 Definitions-descriptions and definitions-requirements in logic

Definitions that solve the problem of describing some objects are usually called real. Definitions expressing the requirement of what the objects should be are called nominal. Sometimes, however, nominal definitions do not mean all definitions-requirements, but only definitions that introduce new language expressions into the language or clarify existing ones. This understanding is not, however, sufficiently consistent: it does not make it possible to unambiguously distinguish between nominal definitions and real ones.

We are entitled to demand from real definitions that they give a true description of reality, to be true. Nominal definitions, like all other requirements, are neither true nor false. Successful nominal definition is characterized as effective, expedient, etc.

Although the distinction between definitions-descriptions and definitions-requirements is undoubtedly important, it is usually not easy to draw. Often the statement in one context sounds like a real definition, and in the other it performs the function of nominal. Sometimes a real definition describing any objects acquires a connotation of the requirement how to use the concept correlated with them. Nominal definition may bear the echo of the description.

Graphic figures are known from psychology that, when closely examined, appear either convex or now concave. Similarly, the same definition, if you think about it, can seem like a description, then a requirement.