82. Definitions of Words and Definitions of Things. -- (1) The verbal definition explains the signification, etymological or conventional, of a word. Its purpose is to make our ideas clear and avoid equivocation.

(2) The real definition says what a thing is. The real definition is essential, natural, or descriptive.

(a) Essential definition. -- To know in a perfect manner what a thing is, is to know its intimate nature, its essence. Now the individual essence, by reason of which this individual subject is what it is, distinct from other individuals, is unknowable to man.^{1} We know only by classes.

(b) Accidental definition -- Neither do we immediately attain the generic or specific essence. On observing the qualities of beings, we do not even know, at first, whether they are natural or accidental; the designation is often only a description, improperly called a descriptive definition.

It is accidental when it designates a thing by means of accessory notes the whole sum of which belongs to this thing alone.

(c) Natural definition. -- When, through induction, the mind comes to discern in the thing one or more necessary qualities, it defines the thing by its properties: this is natural definition.

The definitions used in chemistry, mineralogy, botany, zoology, etc., are descriptive, accidental, or at most natural.

The essential definition, then, is an ideal, which is but rarely attained. And yet, it alone is rigorously scientific or philosophic.

How do we form it?

{1} See Psychology.