A man upon the witness stand makes the declaration that he will testify to the truth, the whole truth and nothing but the truth. A logical definition must contain _the species, the whole species_ and _nothing but the species_. If the definition does not include all the species, it is too narrow; while on the other hand, if it includes other species of the genus it is too broad. An excellent test of this second requirement is to interchange subject and predicate. If the interchanged proposition means the same as the original then the conditions have been met. To illustrate: Original――A trigon is a polygon of three angles. Interchanged――A polygon of three angles is a trigon. The very best way of making the definition conform to this rule is to put to oneself these three questions: 1. Does it include all of the species? 2. Does it exclude all other species of the genus? 3. Has it any unnecessary marks? To exemplify: Let us ask the three questions relative to the following logical definitions: (1) A parallelogram is a quadrilateral whose opposite sides are parallel. (2) A bird is a biped with feathers. _Questions_: (1) Does the definition include all the parallelograms? Yes. Does it exclude all other quadrilaterals? Yes. Are there any unnecessary marks? No. (2) Does it include all birds? Yes. Does it exclude all other bipeds? Yes. Any unnecessary marks? No. _Illogical According to the Second Rule._ (1) A man is a vertebrate animal. (Too broad. Does not exclude other species of the genus, such as horses, dogs, etc.) (2) A barn is a building where horses are kept. (Too narrow. Does not include all of the species, such as cow barn.) (3) An equilateral triangle is a triangle all of whose sides and angles are equal. (Equal angles is an unnecessary mark.) _The Foregoing Definitions Made Logical._ (1) A man is a rational biped. (Proximate genus.) (2) A barn is a building where horses and cattle are kept and hay and grain are stored. (3) An equilateral triangle is a triangle all of whose sides are equal. THIRD RULE. _A definition must not repeat the name to be defined nor contain any synonym of it._ A violation of this rule is known as “a circle in defining” (_circulus in definiendo_). There are some exceptions to this rule, as in the case of compound words and a species which takes its name from its proximate genus. To say that a hobby-horse is a horse, or that an equilateral triangle is a triangle, is not only allowable but necessary, that the proximate genus may be used. _The following definitions are illogical according to the third rule_: (1) A teacher is one who _teaches_. (2) Life is the sum of the _vital_ functions. (3) A sensation is that which comes to the mind through the _senses_. FOURTH RULE. _A definition must not be expressed in obscure, figurative or ambiguous language._