(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.