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Cross-division should be distinguished from cross-references, such as one finds in the subject-catalogues of libraries. If one does not know whether to put a book on the history of English philosophy under History' or England' or Philosophy', he can solve the question practically by putting it under one of them, say • England', and then saying under each of the other heads · See also England'.

The blunder of dividing in such a way as to leave out altogether some object that ought to have been included is likely to be the result of haste rather than of confusion. Where one knows all the objects that are to be divided, he must look about carefully to see that none are omitted. Where he does not he must leave room in his scheme of division and subdivision for additions, e.g.:

I. Indo-European

LANGUAGES.

A. Slavonic

s 1. Russian

2. Others ?
B. Græco-Roman
C. Celtic

1. English
D. Teutonic

2. Gernian 3. Dutch

4. Others ? (E. Others ?

II. Semitic
III. Turanean
IV. Others ?

It need hardly be added that a surbordinate class should not contain any objects not included in the class above it. If we are classifying serious works of history it would not do to include Dumas' "Three Musketeers” in one of the subdivisions, even though it contains some historical matter, for it is not a serious work of history.

Students should make themselves familiar with the following names : Genus is the name of a class of objects divided into smaller classes. Each of these smaller classes is called a Species, Where there is a system of divisions and subdivisions

names.

any class is called a species with reference to those above

it, a genus with reference to those below it, e.g., Some technical organisms or living bodies are a species of

bodies, but are a 'genus' of which animals are a species. In such a system the highest class of all is called the Summum Genus; the lowest, the Infima Species. “When a thing is so peculiar and unlike other things that it cannot easily be brought into one class with them, it is said to be sui generis, or of its own genus ”';* e.g., the rings around Saturn amongst heavenly bodies, the ornithorhyncus and amphioxus amongst animals. The qualities peculiar to the members of some one species or genus, and in virtue of which that species or genus is distinguished from other species or genera, are called the Differentiæ of the species or genus. Thus the possession of reason, by which men may be distinguished from the lower animals, can be called the differentia of the human species. Qualities peculiar to a species, but not used in defining it or distinguishing it from other species, are called Properties, or Propria. Thus laughter is a proprium of human beings. Qualities or states not characteristic of a species are called Accidents.

But every

tion and its purposes.

So far we have taken up the purely formal or negative aspect of division, and pointed out the blunders which any

system of division should avoid. Scientific

actual system of division has a purpose, and to classifica

attain a purpose it is necessary to move in some

definite direction, not merely to avoid the pitfalls on the road. A scientific classification is nothing more than a system of division carried out in such a way as to best serve a given purpose.

One purpose served by classification is to enable a person to find any given object with the least possible trouble. If I have only two or three dozen books it is not worth while to arrange them on the shelf in any particular order, for I can

* Jevons.

In this way

always find the one I want at a single glance. But if I have a library full of books and pamphlets, and I want to be sure of finding one of them at a moment's notice, I must arrange them alphabetically, or topically, or in some other fixed order. When the things themselves cannot be arranged according to any plan, the next best thing is to arrange their names in some fixed order—usually alphabetical—and after the name write where the thing is to be found. we construct directories and gazetteers and the indexes of books. U A second purpose served by classification is to give an easy means of identifying an object when it is found. A suspicious looking person is arrested by the police and they wonder who he is; so they turn up their classified list of criminals, looking up (say) the class · Eyes, blue'; then the subdivision *Height, five feet eleven inches'; then the further subdivision · Fingers, tapering'; and so on, till at last they find the photograph or thumb-mark of the individual in question with his name and record. If the descriptions were not classified the work of identifying the man might be endless.

A function similar to these two, yet distinguishable from them, is to enable us to find or identify a given kind of object. Often we do not know what book or man we want to find, but the classified list can tell us. We want some book or other about colonial furniture, and the subject-catalogue names several ; we want a man who can mend china, and the classified list of tradesmen at the end of the city directory gives a number of names and addresses.

In the same way if we find a new kind of plant in the woods and want to know what it is, we use some botanical • key,' in which kinds of plants are identified by a series of obvious characteristics, and discover the name (not of that individual as such, but) of that kind of plant. In the same way also a classified list of symptoms might save a young physician much valuable time in diagnosing a disease.

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A fourth object of classification is to make it easy to deal V at the same time with things that bear any special relation to

each other. To this end we put the things themselves or their names together. A grocer puts in the same basket all the parcels that are to go to the same house ; a lady writes on the same list the names of the people or shops to be visited on the same afternoon. Here the classification is made for an immediate practical purpose—something must be done about each of the people or things in question, and the classified list helps us to do it. But often, and this is the object of classification in science, things or the names of things are put together because it is desirable to think of them not only at the same time but in relation to each other. * Thus chronological tables, maps, and astronomical charts are made not only to show what happened in a given year or the location of a given place, but also to show the general sequence of events at any given period, the general conformation of a country, the general arrangement of a planetary system, In these cases the relations that determine the classification are those of time and space, but they may be anything : degree of scholarship, in arranging a list of students; degree of strength, in a list of acids ; durability, in a list of fabrics or dyes; cause and effect, when the writer on medicine puts together all the causes and symptoms of a given disease ; means and end, when he adds a list of remedies.

Scientific classifications are most concerned with relations of resemblance and contrast. When a naturalist, for example,

* It is rather common to speak as though all classification were of one's ideas or concepts of things, but this is a blunder. A psychologist classifies ideas and feelings when he points out their resemblances and differences; e.g., the difference between a feeling of terror and a feeling of contempt; but a naturalist classifies things. It is the business of a psychologist to observe the difference between thoughts as such; but every other scientist is concerned with the difference between the objects that we think of, not the thoughts themselves. To think of the difference between things is not the same as to have different thoughts of them.

classifies all animals into vertebrates and non-vertebrates, he merely asserts that each of a certain long list of animals has a backbone, and in this respect and certain others that follow from it resembles each of the others and differs from all of those not on the list.

But what points of resemblance and contrast must be regarded, and in what order, if we are to make a classification scientific? The answer to this is that no basis of classification—no fundamentum divisionisis any better than any other in itself. The only general rule is to choose and arrange fundamenta divisionis in the way that best shows the points of resemblance and contrast in which we are interested or likely to be interested. It is just as scientific to classify books by their size, publishers, date, color, type, or language as by their author or subject-matter, and a great deal more so if we are interested in the former and not in the latter.

If we are not interested in any special characteristic of the objects classified the only thing left is to try to get into the same class objects that have a great many things in common and into different classes objects that have very little in common, SO " that we shall be enabled to make a maximum amount of aggregate assertion with a minimum number of propositions."'* To do this we must choose as our first fundamentum divisionis that point on which the greatest number of other noteworthy points of difference depend. It is better to divide all living things into animals and plants than into those which weigh more than a pound and those which weigh less; because the possession of sensation and the power of spontaneous motion t which distinguish animals from plants involve innumerable other points of difference, while the size involves little or nothing more : to say that a

* Venn's “Empirical Logic,” Chap. XIII., to which the reader is referred. (Macmillan.)

+ This of course is the popular distinction. If we wish to be more scientific we should say the fact that they require protoplasmic foodstuff, or cannot decompose carbonic acid gas, which distinguishes,' etc.

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