The logical process of defining consists of two steps: first, stating the class or group to which the object of definition belongs, as to say that Switzerland is a _country_, the forward pass is a _strategic device in football_, humanism is a _philosophy of personal development_; and second, pointing out the difference between the object of definition and other members of the class, showing how it is distinguished from them. Since the purpose of definition is to limit the thing defined, the practical value of the first step is at once apparent. If, in total ignorance, a resident of India asks you, "What is ragtime?" the most helpful thing in the world that you can do for him is to cleave away with one stroke everything else in the world but music--absolute exclusion of all other human interests--and place ragtime in that comparatively narrow field. That is the first thing of great help. However many qualities you may attribute to ragtime,--whether you call it inspiring, invigorating, pleasing, detestable, or what not,--you are making at best only slow progress toward defining, really limiting ragtime. The number of pleasing things, for example, is so endless, and the things are so diverse in character that your listener is almost as ignorant after such a quality has been attributed as he was before. But the moment that you limit ragtime to music you scatter untold clouds of doubt and place the inquirer in the comfortable position of having a fairly large working knowledge. What is left for the inquirer to do is merely to distinguish ragtime from other kinds of music--after all, a rather simple task. Likewise in any definition, such as that of rotation of crops, the first necessity is to place the subject in its proper field, in this case agriculture; the grand piano in the class of musical instruments; the rocking chair in the class of furniture.
Now sometimes the task of discovering to what class your subject belongs is difficult. Is a believer in Unitarianism a Christian? He follows the ethical teachings of Jesus but denies him any special divinity. In this case obviously the question of classification will depend on the definition that we make of Christianity. Is a man who serves the state in legislative or judicial capacity and at the same time writes novels to be called a statesman or a man of letters? Governments have fallen into difficulty with each other over such things as contraband of war, there being great doubt at times whether a particular thing is properly contraband or not. The question is sometimes doubtful--you will be inclined to say, "I don't know what to call this," but in making a definition call it you must. The United States Government, facing the problem of discovering the proper class for frogs' legs, in determining customs duties after much perturbation placed them under the heading "poultry." Ordinarily you will find slight difficulty in determining the class; but in every case you must patiently search until you have found some class into which your subject naturally fits. Until you have done this you obviously cannot set it apart from other members, because you will not really know what the other members are, you will be forced to run through the total list of human ideas and things. Until you know that _oligarchy_ is one form of political society you cannot know whether to set it off from _democracy_ and _monarchy_ or from _Christianity_ and _Buddhism_. First, then, however difficult, discover the class to which your subject belongs. In the following definition of a _clearing-house_, you will find that in the course of time the class to which the subject belongs has changed, has come to include more space, needs a larger fence to surround it, and therefore the definition has been changed.