DEFINITION. — As a logical function, definition (definire: to delimit, to set boundaries) is the act by which thought clarifies and determines the content of a concept, resolving it into its essential constituent elements; objectively, it is this conceptual determination in which is expressed what the concept is, and therefore also what the thing signified by it is. Indeed, the concepts with which human thought expresses reality usually present themselves as indistinct wholes that require further analysis and discernment to ascend to the level of clear scientific knowledge. Thus, definition comes to stand, in relation to the notion defined, as a distinct and scientific concept in contrast to a confused and initial concept.
In logic, various kinds of definition are usually distinguished, though these are not universally accepted without distinction, given the fluctuating terminology and the differing metaphysical and epistemological presuppositions involved. Traditional logic distinguishes verbal definition, which is a simple explanation of a word; nominal definition, which is an initial clarification of a thing drawn from the etymological meaning of the word; and real definition, which is an expression of what a thing is through the determination of the objective content of the concept. Real definition is further subdivided, in descending gradation of perfection and ontological penetration, into essential (the only definition properly so called), causal, genetic, and descriptive.
A terminology deriving from Kant divides definition into synthetic, which is achieved through a constructive movement of a new total concept from elements or parts, and analytic, which resolves a given concept into its partial notes.
Socrates was the first to highlight the importance of definition, making it consist in the purpose of science to determine the exact value of concepts, that is, what each thing is. This notion of definition as the determination of the essence of a thing is found in Plato (Theaet., 209 A), and particularly in Aristotle, who developed a systematic theory of definition, distinguishing between real and nominal definition, establishing the fundamental laws of essential definition and the related methods of inquiry (cf. Anal. post., II, 3, 7, 13; Top., I, 5, 8; VI, 4; De anim., II, 2, 1). Scholasticism, developing it further, retained Aristotle’s theory; St. Thomas Aquinas: “definitio indicat rei quidditatem et essentiam” (Sum. Theol., 2a-2a, q. 4, a. 1, in c.).
In modern logic, amidst the great variety of terminology and assessment of the scientific value of definition, the tendency to deny the distinction between nominal and real definition has grown stronger. For Pascal, Reid, and Stuart Mill, definitions are nothing more than explanations of words. Others consider definition a mere clarification of concepts, with only methodological value. It is undoubtedly true that every definition is first and foremost a determination of a concept by means of other concepts, and necessarily also an explanation of the word by which that concept is expressed; but this does not negate the fact that definition can have a bearing on the clarification and determination of reality itself. Indeed, granted the objectivity of our concepts, to the extent that this is true, every objective determination of a concept is also a cognitive determination of reality in its essential contents, however more or less distinctly they may be given in the concept. Even so, it cannot and should not be demanded of definition that it provide an exhaustive logical expression of reality.
Logic establishes various laws or norms for a good definition, the principal of which requires that the definition be adequate, that is, one that applies to all and only the subjects comprehended under the defined concept; and consequently, with regard to essential definition, that it express the proximate genus and the specific difference (v. PREDICABILI).