The concept is a _general idea_ of a class of things; the _term_ is the expression of that general idea. The concept is the idea of a class of things; the term is the _label_ affixed to the thing. To illustrate this last distinction, let us take the concept and term of "bird," for instance. By perception, comparison, and classification of the qualities of living things we have arrived at the conclusion that there exists a great general class the qualities of which may be stated thus: "Warm-blooded, feathered, winged, oviparous, vertebrate." To this general class of quality-possessing animals we apply the English term "bird." The name is merely a symbol. In German the term is _vogel_; in Latin, _avis_; but in each and every case the _general idea_ or _concept_ above stated, _i.e._, "warm-blooded, feathered, winged, oviparous, vertebrate," is meant. If anything is found having all of those particular qualities, then we know it must be what we call a "bird." And everything that we call a "bird" must have those qualities. The term "bird" is the symbol for that particular combination of qualities existing in a thing. There is a difference between a mental image of the imagination and a concept. The mental image must always be of a _particular_ thing, while the concept is always an idea of a _general class_ of things which cannot be clearly pictured in the mind. For instance, the imagination may form the mental picture of any known bird, or even of an imaginary bird, but that bird always will be a distinct, _particular_ bird. Try to form a mental picture of the general class of birds--how will you do it? Do you realize the difficulty? First, such an image would have to include the characteristics of the large birds, such as the eagle, ostrich, and condor; and of the small birds, such as the wren and humming bird. It must be a composite of the shape of all birds, from the ostrich, swan, eagle, crane, down to the sparrow, swallow, and humming bird. It must picture the particular qualities of birds of prey, water birds, and domestic fowls, as well as the grain eaters. It must exhibit all the colors found in bird life, from the brightest reds and greens down to the sober grays and browns. A little thought will show that a clear mental image of such a concept is impossible. What the most of us do, when we think of "bird," is to picture a vague, flying shape of dull color; but when we stop to think that the term must also include the waddling duck and the scratching barnyard chicken, we see that our mental image is faulty. The trouble is that the term "bird" really means "all-bird," and we cannot picture an "all-bird" from the very nature of the case. Our terms, therefore, are like mathematical figures, or algebraic symbols, which we use for ease, speed, and clearness of thinking.