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."]
pg002 1/2
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