A violation of this rule is referred to in logic as “defining the unknown by the still more unknown” (_ignotum per ignotius_). It is known that the purpose of definition is to make clear some obscure term, consequently unless every word used is understood the chief aim of the definition has been defeated. From this it must not be inferred that all definitions should be free from technical terms. Such a restriction would make the defining of many terms unsatisfactory and in a few cases practically impossible. To the student of evolution the following definition by Spencer is intelligible while to the uninitiated it would appear obscure: “Evolution is a continuous change from an indefinite, incoherent homogeneity to a definite coherent heterogeneity through successive differentiations and integrations.” This rule insists upon simple language when it is possible to use such in giving an accurate and comprehensive meaning to the term defined. _Illogical Definitions According to the Fourth Rule._ (1) “A net is something which is reticulated and decussated, with interstices between the intersections.” Dr. Johnson. (2) “Thought is only a cognition of the necessary relations of our concepts.” (3) “The soul is the entelechy, or first form of an organized body which has potential life.” Aristotle. FIFTH RULE. _When possible the definition must be affirmative rather than negative._ The fact that there are a considerable number of terms which admit of a negative definition only, takes from the force of this rule. Such terms as deafness, inexpressible, infidel and the like can best be defined negatively. It likewise happens that when words are used in pairs it is expedient to define one affirmatively and the other negatively. Recall, for example, the definitions of relative and absolute terms: “A relative term is one which needs another term to make its meaning clear.” “An absolute term is one which does not need another term to make its meaning clear.” _Illogical Definitions According to the Fifth Rule._ (1) A gentleman is a man who is not rude. (2) An element is a substance which is not a compound. (3) An univocal term is a term which does not have more than one meaning. =8. TERMS WHICH CANNOT BE DEFINED LOGICALLY.= A logical definition insists upon a proximate genus and differentia. But as there is no genus higher than the highest genus (_summum genus_) then surely such cannot be defined logically. The words _being_ and _thing_ illustrate terms of this class. Moreover, it is impossible to give a satisfactory definition of an individual (_infima species_) as no attributes can be mentioned which will distinguish definitely and permanently the individual from others of the class. We may perceive the attributes but not those that are possessed solely by the individual. To say that Abraham Lincoln was a man who was simple and honest is not a definition, as other men have had the same characteristics.