A concept, once formed, refers not merely to the particular concretes which happen to give rise to it, but to all
concretes possessing the distinguishing characteristic(s) involved—all concretes of this kind that now exist, ever
did exist, or ever will exist.
The first level of man's concepts involves the integration into distinct classes of perceptually-observable concretes.
This level provides the base for the much more complex and far-reaching structure of concepts that rises from it.
On the one hand, man proceeds to integrate his narrower concepts into wider concepts (again, by isolating and
integrating distinguishing characteristics), and then his wider concepts into still wider concepts. On the other hand,
he proceeds to refine his knowledge by subdividing wider concepts into narrower classifications or categories.
An example of the first process, the integration of narrower concepts into wider ones, may be observed when man
moves from the concepts of "chair," "table," and "bed" to the concept "furniture"—then, by integrating such
additional concepts as ''household goods"—then, by integrating such additional concepts as "automobile," reaches
the still wider concept of "manufactured utilitarian objects." An example of the second process, the subdividing of
wider concepts into narrower ones, may be observed when man moves from the general concept of "tree" to the
classification of various types of trees, such as "oak," "birch," "maple," etc.
As we have noted, the tool that makes it possible for man to retain and designate his concepts is language.
Language consists of an organized system of auditory-visual symbols by means of which man retains his concepts
in firm, precise form. By the use of words, i.e., by means of single units that stand for unlimited numbers of
particulars, man's mind is able to hold and work with wide categories of entities, attributes, actions, and
relationships—a feat that would not be possible if he had to form images of each concrete subsumed under those
categories. Words enable man to deal with such broad, complex phenomena as "matter," "energy," "freedom,"
"justice"—which no mind could grasp or hold if it had to visualize all the perceptual concretes these concepts
designate.
Not only does man need symbols to retain and designate his concepts, but he specifically needs an organized
system of linguistic symbols. A random collection of images or other non-linguistic symbols could never permit the
exactitude, clarity—and complexity—his thinking requires.