Definition and Some of its Difficulties

If our thought is to be clear and we are to succeed in communicating it to other people, we must have some method of fixing the meanings 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 the 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' indicates the particular property that distinguishes whales from other such marine animals as fishes, seals, jelly-fish, lobsters, etc. 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, etc. Similarly we may say that religion is such a system as Christianity, Islam, Judaism, Christian Science, etc. 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. It is, however, a useful way of supplementing a definition if the definition itself is definite without being easily understandable. If, for example, we explain what we mean by religion by saying: 'A religion is a system of beliefs and practices connected with a spiritual world, such as Christianity, Islam, Judaism, Christian Science, and so on', we may succeed in making our meaning more clear than it would be if we had been given the definition alone.

Failure of an attempt at definition to serve its purpose may result from giving as distinguishing mark 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. Thinking, for example, of the most obvious difference between a rabbit and a cabbage, we might be tempted to define an animal as a living organism which is able to move about. This would combine both faults mentioned above, since some animals (e.g. some shell-fish such as the oyster) are not able to move about for the whole or part of their lives, while some vegetables (such as the fresh-water alga Volvox) do swim about. Of course, anyone who used the above definition might claim to be defining 'animal' in a new and original way to include Volvox and exclude oysters, but he would have failed to produce a definition which defined the ordinary use of the word 'animal'.

More commonly an attempt at definition fails by not indicating correctly the general class to which the thing defined belongs. One meets, for example, in psychological writings such definitions as: 'Intelligence is a state of mind characterized by the ability to learn and solve problems.' The second part of the definition is all right, but the word 'intelligence' is not used for a state of mind; and the person who defines 'intelligence' like this does not in his actual use of the word make it stand for a state of mind. Such conditions as despair, concentration, alertness, and hope can be called 'states of mind'. 'Intelligence' is used for a quality of mind, not for a state. If the word 'quality' replaced the word 'state' in the above definition it would indicate very well the current use of the word 'intelligence'.

(From Straight and Crooked Thinking, by R. H. Thouless.)