DEFINITION

A mental process, namely, one of clarifying the meaning of a term by analyzing and relating the elements involved in it; or, alternatively, the product of a mental process, viz, an expression explaining the use of the term or its meaning. Definition is the opposite of divi sion, which separates the elements involved in the meaning of a term or in the thing it signifies.

Notion. As a mental process, definition arises not only from simple apprehension, but also from judg ment and reasoning, and, in the latter case, through both deduction and induction. Thus intellectual apprehension of one's experiences with individual human beings can lead one to realize that "animal in whom instinct is replaced by reason" is what is meant by "man." The process here is often one of defining through division, as exemplified in Plato's Sophist (218D–221C). Or, one may judge it true to say, "Man is an animal," and so utilize the notion of "animal" as a clarification of "man." Finally, one may argue to the conclusion, "Man is meant to live in society," from the findings of psychological tests, and then employ "social" as a clarification of "man." This method is often used by St. thomas aqui nas in his Summa theologiae, as when he formulates the full definition of a virtue only at the end of its treatment (for example, that of charity, ST 2a2ae, 23). Definitions can thus be said to grow or expand as one's knowledge is increased and is related consciously to what was previously known. As the product of a mental process, definition is distinguished from the term or thing defined, which is often labeled "the definitum," or "the definiendum." In the examples cited above, "man" is the definitum. Furthermore, definition is not a sentence or proposition, but an expression that merely juxtaposes the definitum and the definition; this can be signified by means of a colon—"man: social being." For this reason, definitions are not true and false, but good and bad or adequate and inadequate. For it is generally agreed that definition, as the product of a mental process, is related only to the first act of the mind, that is, to simple apprehension.

Kinds. There are two major groups of definitions. The first includes expressions that explain the use of a term; these are called nominal definitions because the definitum is a term usually in the noun form (Lat. nomen ). There are four classes of nominal definition. The first makes use of a synonym or of the corresponding word in another language, as in "man: human being," or "man: hombre. " Such a definition is of value only when the definition is clearly more known than the definitum. The second employs the etymology of a term—"man: from the Anglo-Saxon mann, " or "magnolia: from Magnol, its discoverer's name." The third is based on the history of a term's use throughout the ages; thus one may study the usage of "man" in Chaucer, Shakespeare, Restoration drama, etc. The fourth is based on an imposed or stipulated use of a term, as is done when proving theorems in mathematics and symbolic logic.

The second major group of definitions includes all expressions that explain the meaning of the term and of the concept it signifies. Aristotle explains it as a phrase signifying a thing's essence (Topics, A 101b). This kind of definition is often called "real," not because the nominal definition is fictional, but because the definitum of the second group is taken as referring more directly to reality (Lat. res ).

There are many varieties of real definition. First, a thing may be defined in terms of its efficient causes, or its origins, as in "man: one born of rational parents," or "water: a product of oxygen and hydrogen." Second, one may define something in terms of its end or purpose, as in "man: a being whose destiny it is to rule the universe," or "watch: an instrument for telling time." Third, a thing may be defined in terms of its intrinsic principles, as in "body: a composite of primary matter and substantial form," or "man: a composite of organic body and intellectual soul." This is sometimes labeled a physical definition. Fourth, a definition may be given through a thing's properties and accidents, as in "man: a being capable of enjoying a joke," or "water: a substance with a boiling point of 100°C." This is often called a descriptive definition. Finally, a thing may be defined in terms of its genus and difference, as in "man: rational animal." This is called a metaphysical definition.

These definitions are not all of equal value. The ordinary man tends to define in terms of synonyms, purposes and accidents. The mathematician finds use only for stipulated definitions. The philosopher and the scientist employ all forms of real definition, together with the results of etymological and historical studies, often combining these into one definition that completely expresses the essence of the definitum, as in "man: one born of rational parents, composed of organic body and intellectual soul, made to dominate the universe, meant to live in society, etc."

Rules. In order to bring order and accuracy to the work of defining, logicians have developed several rules for formulating definitions. These may be summarized as follows. First, the definition should be coextensive with the definitum. Thus to define man as "two-legged" is bad form, for other things besides man are two-legged. Second, the definitum should never be found in the definition, as in "man: creature composed of organic body and human soul;" here "human" is equivalent to "man." This is perhaps the most common form of bad definition. Third, whenever possible the definition should be in univocal terms, so as to avoid ambiguity. Fourth, whenever possible the definition should be in positive, affirmative terms, since negative terms do not tell what a thing is but only what it is not. Thus, to define man as "a non-feathered creature" is to say little about him. Finally, every definition should contain a genus and a difference, the genus expressing what the definitum has in common with other things or words, and the difference expressing what is peculiar to the definitum. It may be noted that, while all definitions should indicate a genus and a difference, the best definition will specify the proximate genus and the specific difference, for it is this kind of definition that fulfills perfectly the logical rules cited above. But such a perfect definition is rare (the standard example, "man: rational animal," may well be unique), presupposing, as it does, extensive investigation of its subject matter.

Real definitions that comply with all the rules for definition are difficult to obtain in theology since analogical and negative terms are the best available for defining supernatural entities. Nor do existentialist philosophers feel at home with the notion of a real definition that is oriented to essence. Nevertheless, scientists and philosophers recognize in definition a valid mode of attaining science through continued clarification of terms and their meanings.

See Also: quiddity; species; distinction, kinds of.

Bibliography: The Material Logic of John of St. Thomas, tr. y. r. simon et al. (Chicago 1955). f. h. parker and h. b. veatch, Logic as a Human Instrument (New York 1959). j. a. oesterle, Logic: The Art of Defining and Reasoning (2d ed. Englewood Cliffs, N.J. 1963). v. e. smith, The Elements of Logic (Milwaukee 1957); "Definitions," in From an Abundant Spring (New York 1952), The Walter Farrell Memorial v. of The Thomist, 337–362. j. a. mourant, Formal Logic (New York 1963). e. d. simmons, The Scientific Art of Logic (Milwaukee 1961).

[e. bondi]