DEFINITION (definio, to mark out limits), "is used in Logic to signify an
expression which explains any term so as to separate it from everything else, as
a boundary separates fields" (Whately). A Definition is a categorical
proposition, consisting of two parts or members, viz., a subject defined
(membrum definitum) and the defining attributes of the subject,
i.e., those by
which it is distinguished from other things (membrum difiniens).
Logicians distinguish definitions into Nominal and Real. Those are called
Nominal which explain merely the meaning of the term; and
real, which explain
the nature of the thing signified by the term. See Whately, Logic, bk. II. chap.
V. sec. 6.
"By a real, in contrast to a verbal or nominal definition, the logicians do not
intend 'the giving an adequate conception of the nature and essence of a
thing;' that is, of a thing considered in itself, and apart from the conceptions
of it already possessed. By verbal definition is meant the more accurate
determination of the signification of a word; by real the more accurate
determination of the contents of a notion. The one clears up the relation of
words to notions; the other of notions to things. The substitution of
for real would, perhaps, remove the ambiguity. But if we retain the term
the aim of a verbal definition being to specify the thought denoted by the word,
such definition ought to be called notional, on the principle on which the
definition of a notion is called real; for this definition is the exposition of
what things are comprehended in a thought" (Hamilton, Reid's Works, p. 691,
On the question whether logical Definition is real or nominal,
various views are held. On the one hand, e.g., Mansel says:—"In the sense in
which nominal and real definitions were distinguished by the scholastic
logicians, logic is concerned with real, i.e., notional definitions only; to
explain the meaning of words belongs to dictionaries or grammars" (Prolegom.,
Logic, p. 189). Whately, on the other hand, holds that "Logic is concerned with
nominal definitions alone" (Logic, bk. II. ch. V. sec. 6). Mill also says—"The
simplest and most correct notion of a definition is a proposition declaratory of
the meaning of a word" (Logic, bk. I. ch. VIII. sec. 1). Accordingly he
considers a Definition a "purely verbal" proposition.
"There is a real distinction between definitions of names and what are
erroneously called definitions of things; but it is that the latter, along with
the meaning of a name, covertly asserts a matter of fact. This covert assertion
is not a definition, but a postulate.
The definition is a mere identical
proposition, which gives information only about the use of language, and from
which no conclusions respecting matters of fact can possibly be drawn. The
accompanying postulate, on the other hand, affirms a fact which may lead to
consequences of every degree of importance. It affirms the real existence of
things, possessing the combination of attributes set forth in the definition,
and this, if true, may be foundation sufficient to build a whole fabric of
scientific truth" (Mill, Logic, bk. I. ch. VIII. sec. 6).
Real definitions are sometimes divided into essential and accidental. An
essential definition states what are regarded as the constituent parts of the
essence of that which is to be defined; and an accidental definition (or
description) lays down what are regarded as circumstances belonging to it, viz.,
properties or accidents, such as causes, effects, &c. But in reality all
Definition is essential, and hence is not to be confused with Description
(q.v.). The Definition is an account of the essence of the notion or thing;
hence it must contain the genus proximum and the differentia. For various other
classifications of Definitions, see Ueberweg's System of Logic, p. 164,
The Faults of Definition are thus enumerated by Ueberweg (Logic, p. 172, Lindsay's transl.):—(1) Too great
width or narrowness; (2)
Redundancy, or the mention of derivative determinations or properties, besides
the essence; (3) Tautology, when the notion to be defined is repeated in the
Definition; (4) Circulus in Definiendo, or the attempt to define a notion by
means of those notions which presuppose it; (5) Definition by figurative
expression or by mere negatives.
Aristotle, Topic, lib. VI.; Poster. Analyt., lib. II. ;
Port Royal Logic, part I. ch. XII., XIII., XIV.; part II. ch. XVI.; Locke,
Essay on Human
Understanding, bk. III. ch. III. and IV.; Reid, Account of Aristotle's Logic, ch.
II. sec. 4.