84. Rules of Definition. -- These relate to the twofold function of definition.

I. First point of view: The definition must furnish the first principles of the science. Hence the following rules:

(1) The definition must proceed from an object antecedent to the thing defined. Consequently:

(a) Correlative terms (as health and sickness), being simultaneous, cannot be used to define each other.

(b) Different members of a division are not defined by one another.

(c) A thing is not defined by itself or by that to which it is antecedent.

(2) The genus used in the definition must be the nearest genus.

II. Second point of view: The definition should help to clearness of ideas. It must be clearer than the thing to be defined. Consequently, it must:

(a) Not repeat the name of the thing to be defined.

(b) Avoid metaphorical, ambiguous, and obscure terms.

(c) Be concise and adequate.