Few exercises are better adapted to the development of powers of discrimination and precision than practice in defining logically the common terms of every-day life. For example: “A book is a pack of paper-sheets bound together.” “A chair is a piece of furniture with back and seat, designed for the seating of one person.” “A lead pencil is a cylindrical writing implement with lead through the center.” “A door is an obstacle designed to swing in and out to open and close an entrance.” “An eraser is an implement made to rub out written or printed characters.” These definitions, coming from training school students, are not above criticism, yet they illustrate the point in hand. =7. THE RULES OF LOGICAL DEFINITION.= Five rules summarize the requirements to which a logical definition must conform. FIRST RULE. _A logical definition should state the essential attributes of the species defined._ This means that a logical definition should contain the species, the proximate genus and the differentia. As the terms species, genus and differentia have been explained, it will be sufficient to briefly illustrate this rule. _Logical According to the First Rule._ species genus differentia (1) A ――――――― is a ――――― with ―――――――――――. bird biped feathers species genus differentia (2) A ――――――― is a ―――――― ―――――――――――――――――――――――――――. mascot person supposed to bring good luck species genus differentia (3) ―――――――― is a ―――――― of ―――――――――――――――――. Religion system faith and worship species genus differentia (4) A ―――――――― is a ―――――――――――― ―――――――――――――. moonbeam ray of light from the moon _Illogical According to the First Rule._ (1) A man is a rational animal. (“Biped” is the proximate genus, not “animal.”) (2) A Connotative term always denotes both an object and an attribute. (No genus.) (3) A trigon is a polygon. (No differentia.) (4) It is a term which denotes an indefinite number of objects or attributes. (No species.) _The Foregoing Illogical Definitions Made Logical._ (1) A man is a rational _biped_. (2) A Connotative term is a _term_ which denotes both an object and an attribute. (3) A trigon is a polygon of _three angles_. (4) A _general term_ is a term which denotes an indefinite number of objects or attributes. SECOND RULE. _A logical definition should be exactly equivalent to the species defined._ This means that the species must equal the genus plus the differentia or the subject and predicate of the definition must be co-extensive――of the same bigness. The subject must refer to the same number of objects as the predicate.