Chapter 26

_CLASSIFICATION._

'Classification,' or the formation of Classes, is a Mental Process, in which we imagine that we have put together, in a group, certain Things. Such a group is called a '=Class=.'

This Process may be performed in three different ways, as follows:--

(1) We may imagine that we have put together all Things. The Class so formed (i.e. the Class "Things") contains the whole Universe.

(2) We may think of the Class "Things," and may imagine that we have picked out from it all the Things which possess a certain Adjunct _not_ possessed by the whole Class. This Adjunct is said to be '=peculiar=' to the Class so formed. In this case, the Class "Things" is called a '=Genus=' with regard to the Class so formed: the Class, so formed, is called a '=Species=' of the Class "Things": and its peculiar Adjunct is called its '=Differentia='. pg002 As this Process is entirely _Mental_, we can perform it whether there _is_, or _is not_, an _existing_ Thing which possesses that Adjunct. If there _is_, the Class is said to be '=Real='; if not, it is said to be '=Unreal=', or '=Imaginary=.'

[For example, we may imagine that we have picked out, from the Class "Things," all the Things which possess the Adjunct "material, artificial, consisting of houses and streets"; and we may thus form the Real Class "towns." Here we may regard "Things" as a _Genus_, "Towns" as a _Species_ of Things, and "material, artificial, consisting of houses and streets" as its _Differentia_.

Again, we may imagine that we have picked out all the Things which possess the Adjunct "weighing a ton, easily lifted by a baby"; and we may thus form the _Imaginary_ Class "Things that weigh a ton and are easily lifted by a baby."]

(3) We may think of a certain Class, _not_ the Class "Things," and may imagine that we have picked out from it all the Members of it which possess a certain Adjunct _not_ possessed by the whole Class. This Adjunct is said to be '=peculiar=' to the smaller Class so formed. In this case, the Class thought of is called a '=Genus=' with regard to the smaller Class picked out from it: the smaller Class is called a '=Species=' of the larger: and its peculiar Adjunct is called its '=Differentia='.

[For example, we may think of the Class "towns," and imagine that we have picked out from it all the towns which possess the Attribute "lit with gas"; and we may thus form the Real Class "towns lit with gas." Here we may regard "Towns" as a _Genus_, "Towns lit with gas" as a _Species_ of Towns, and "lit with gas" as its _Differentia_.

If, in the above example, we were to alter "lit with gas" into "paved with gold," we should get the _Imaginary_ Class "towns paved with gold."]

A Class, containing only _one_ Member is called an '=Individual=.'

[For example, the Class "towns having four million inhabitants," which Class contains only _one_ Member, viz. "London."] pg002½ Hence, any single Thing, which we can name so as to distinguish it from all other Things, may be regarded as a one-Member Class.

[Thus "London" may be regarded as the one-Member Class, picked out from the Class "towns," which has, as its Differentia, "having four million inhabitants."]

A Class, containing two or more Members, is sometimes regarded as _one single Thing_. When so regarded, it may possess an Adjunct which is _not_ possessed by any Member of it taken separately.

[Thus, the Class "The soldiers of the Tenth Regiment," when regarded as _one single Thing_, may possess the Attribute "formed in square," which is _not_ possessed by any Member of it taken separately.]

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