If our thoughts is to be clear and we are successful in communicating it to other people we must have some method of fixing the meaning of the words we use. When we use a word whose meaning is not certain we may well be asked to define it. There is a useful traditional device for doing this by indicating the class to which whatever is indicated by the term belongs and also other particular property which distinguishes it from all other members of the same class thus we may define a whale as a marine animal that spouts. Marine animal in this definition indicates the general class to which the whale belongs and spouts indicate the particular property that distinguishes whales from other such marine animals as fishes, seals, jellyfish, and lobsters. In the same way we can define an even number as a finite integer divisible by two or a democracy as a system of government in which the people themselves rule.There are other ways of course of indicating the meanings of words we may for example find it hard to make a suitable definition of the word animal, so we say that an animal is such a thing as a rabbit, dog , fish or goat. Similarly we may say that religion is such a system as Christianity, Islam Judaism and Buddhism. This way of indicating the meaning of a term by enumerating examples of what it includes is obviously of limited usefulness. If we indicated our use of the word “animal” as above our hearers might for example be doubtful whether a sea-anemone or a slug was to be included in the class of animals. Its however a useful way of supplementing a definition if the definition itself is definite without being easily understandable. Failure of an attempt at a definition to serve its purpose may result from giving as distinguishing ark one which either does not belong to all the things the definition is intended to include, or does belong to some members of the same general class which the definition is intended to exclude. Adapted from straight and crooked thinking, by R.H .Thouless
