ozen other qualities are likewise found not to belong in common to
_all_ dogs, and so have had to go; and all that is left to his _dog_ is
_four-footedness_, and a certain general _form_, and a few other dog
qualities of habit of life and disposition. As the term _dog_ has been
gaining in _extent_, that is, as more individuals have been observed and
classed under it, it has correspondingly been losing in _content_, or it
has been losing in the specific qualities which belong to it. Yet it
must not be thought that the process is altogether one of elimination;
for new qualities which are present in all the individuals of a class,
but at first overlooked, are continually being discovered as experience
grows, and built into the developing concept.

DEFINITION OF CONCEPT.--A concept, then, is _our general idea or notion
of a class of individual objects_. Its function is to enable us to
classify our knowledge, and thus deal with classes or universals in our
thinking. Often the basis of a concept consists of an _image_, as when
you get a hazy visual image of a mass of people when I suggest _mankind_
to you. Yet the core, or the vital, functioning part of a concept is its
_meaning_. Whether this meaning attaches to an image or a word or stands
relatively or completely independent of either, does not so much matter;
but our meanings must be right, else all our thinking is wrong.

LANGUAGE AND THE CONCEPT.--We think in words. None has failed to watch
the flow of his thought as it is carried along by words like so many
little boats moving along the mental stream, each with its freight of
meaning. And no one has escaped the temporary balking of his thought by
failure to find a suitable word to convey the intended meaning. What
the grammarian calls the _common nouns_ of our language are the words by
which we name our concepts and are able to speak of them to others. We
define a common noun as "the name of a class," and we define a concept
as the meaning or idea we have of a class. It is easy to see that when
we have named these class _ideas_ we have our list of common nouns. The
study of the language of a people may therefore reveal much of their
type of thought.

THE NECESSITY FOR GROWING CONCEPTS.--The development of our concepts
constitutes a large part of our education. For it is evident that, since
thinking rests so fundamentally on concepts, progress in our mental life
must depend on a constant growth in the number and charac