Chapter 8

A definition is a concise statement setting forth the nature of the thing in question, that is to say, such statement as expresses in brief the nature of the thing in question. For example, man is a rational mortal animal capable of intelligence and knowledge. Now, many men have discoursed at length on the nature of man, that is, they have written long and extensive treatises on the subject. But these are not concise and, therefore, arc not definitions. There are also consise statements, such as apophthegms, but, since they do not .set forth the nature of a thing, they are not definitions. A name, too, oftentimes indicates the nature of the thing in question, but it is not a definition. For the name is one word, while the definition is a statement, and a statement is made up of at least two words. (Therefore, the definition is a name explained, whereas a name is a term of a proposition, when it is in conjunction.)

The definition is made up of genus and constituent, that is to say, essential differences. Thus it is with the definition of animal, for animal is an animate sentient substance. Here the genus is substance, while the constituent differences are the being animate and sentient. The definition may also be taken from matter and form, as, for example: A statue is that which is made of bronze and represents the form of a man. In this case the bronze is the matter, while the representation of the shape of the man is the form of the statue. The matter corresponds to the genus and the form to the specific difference. The definition may also be taken from subject and purpose. Medicine, for example, is concerned with human bodies and is productive of health. Here the subject of medicine is the human body, whereas its purpose is health.

Now, the description is made up of non-essential elements, that is to say, of properties and accidents. For example, man is an animal which is able to laugh, walks erect, and has broad nails. These elements are non-essential. For this reason it is called description, since it outlines, bringing out not the essential substance but only the things consequential to it. The descriptive definition is a combination of essentials and non-essentials, as, for example: Man is rational animal walking erect and having broad nails.

Definition is the term for the setting of land boundaries taken in a metaphorical sense. For, just as the boundary separates that which belongs to one from that which belongs to another, so does the definition set off the nature of one thing from that of any other.

Now, the soundness of a definition lies in its having neither too few nor too many terms, while its vice lies in its having either too few or too many terms. A perfect definition is one which is convertible with the thing defined, while an imperfect one is one which is not. Neither is that which has too few terms convertible (nor that which has too many), for, when it has too many terms, it covers too few things, whereas, when it has too few terms, it covers too many things. (And so one may say that nature has discovered a wonderful device—poverty that is wealthy and wealth feigning poverty.) For example, the perfect definition of man is: Man is rational mortal animal. Notice how this is convertible, for every rational mortal animal is a man and every man is a rational mortal animal. Now, if one term were to be left out, the definition would cover too m