NECESSITY OF CLEAR DEFINITIONS. 75 



In every science we should define all the terms, and 

 prove all the propositions. Every scientific term requires 

 to be defined, in order to enable us to distinguish it from 

 all other terms. If such terms were not defined, they 

 would confuse our minds. The limits also of each term 

 are indicated and known by its definition or meaning. A 

 definition of a term or thing should fulfil the following 

 conditions : 



1. It must be clear, and not be expressed in obscure, 



ambiguous, or figurative language. 



2. It must contain neither too little nor too much. 



3. It must be affirmative if possible, because a thing 



. cannot be clearly defined by stating all that it is 

 not ; for instance, c solid ' cannot be clearly defined 

 as 6 that which is not a liquid or gas.' 



4. It must not contain the name of the thing defined, 



because a thing cannot be defined by a repetition 

 merely. 



5. It ought to include all the essential qualities and at- 



tributes of the thing defined, and none of the non- 

 essential ones. In scientific discourse, the same 

 term must have the same meaning attached to it 

 throughout any one chain of reasoning. In pure 

 logic, however, the meaning of the terms we 

 reason about may be unknown, because we employ 

 representative symbols in their stead, and operate 

 not upon ideas merely, but upon forms of 

 thought. 



Words and sounds excite ideas, but no idea exists in 

 the word or sound alone. A term is a mark, used to in- 

 dicate an idea, or represent a thing, and may consist either 

 of one word or several. A single term should properly 



