general
term _mammals_. Likewise with birds, reptiles, insects, and all the
rest. In order to accomplish this, many individuals of each class had to
be observed, the qualities common to all members of the class
discriminated from those not common, and the common qualities retained
as the measure by which to test the admission of other individuals into
this class. The process of classification is made possible by what the
psychologist calls the _concept_. The concept enables us to think
_birds_ as well as bluebirds, robins, and wrens; it enables us to think
_men_ as well as Tom, Dick, and Harry. In other words, _the concept lies
at the bottom of all thinking which rises above the seeing of the
simplest relations between immediately present objects_.

GROWTH OF A CONCEPT.--We can perhaps best understand the nature of the
concept if we watch its growth in the thinking of a child. Let us see
how the child forms the concept _dog_, under which he is able finally to
class the several hundred or the several thousand different dogs with
which his thinking requires him to deal. The child's first acquaintance
with a dog is, let us suppose, with a pet poodle, white in color, and
named _Gyp_. At this stage in the child's experience, _dog_ and _Gyp_
are entirely synonymous, including Gyp's color, size, and all other
qualities which the child has discovered. But now let him see another
pet poodle which is like Gyp except that it is black in color. Here
comes the first cleavage between _Gyp_ and _dog_ as synonyms: _dog_ no
longer means white, but may mean _black_. Next let the child see a brown
spaniel. Not only will white and black now no longer answer to _dog_,
but the roly-poly poodle form also has been lost; for the spaniel is
more slender. Let the child go on from this until he has seen many
different dogs of all varieties: poodles, bulldogs, setters, shepherds,
cockers, and a host of others. What has happened to his _dog_, which at
the beginning meant the one particular little individual with which he
played?

_Dog_ is no longer white or black or brown or gray: _color_ is not an
essential quality, so it has dropped out; _size_ is no longer essential
except within very broad limits; _shagginess_ or _smoothness_ of coat is
a very inconstant quality, so this is dropped; _form_ varies so much
from the fat pug to the slender hound that it is discarded, except
within broad limits; _good nature_, _playfulness_, _friendliness_, and a
d