A particularly characteristic feature of our perception is the grouping together into a mental unit of elements which are not united either spatially by contiguity or nearness, or by similarity of their coloring, or their other attributes. The grouping of such elements into a unitary mental state is often the result of a repeated necessity for reacting upon this sum of impressions by a unitary movement. The newspaper held upside down does not invite the reaction of reading. Parts which are separated by blank spaces or by black bars, are separately perceived. But the words and sentences are not perceived, because we have not previously been obliged to read under such conditions. Looking into a furnished room I perceive at once tables, chairs, and other pieces of furniture, although the legs of a chair, for example, are spatially and by their coloring better connected with the carpet than with the back of the chair. When I am looking at a portrait standing upside down, the dark hair and the dark background become a mental unit, a percept of a dark area. The light face is another mental unit. In upright position the hair separates from the background and unites with the face. I then perceive a person before a dark background, in spite of the similarity of coloring between some parts of the figure and the background, in spite of the difference of coloring between some parts of the figure and other parts. The grouping of the elements in perception is therefore widely different from that which would result from the stimuli directly. It is determined by our habits of reaction upon such groups as frequently appear together in the world in which we live. [Illustration: FIG. 16.--TWO POSSIBILITIES OF PERCEPTION.] Let us illustrate this by two figures. Figure 16 may be perceived as a rabbit’s or as a duck’s head. When we perceive the figure as a rabbit’s head, the white streaks to the right of the eye are two separate sensation groups, each of them unified with respect to the effect produced by them in our nervous system. They are then the animal’s lips. At the same time the protrusions to the left make us conscious of softness, warmth, flexibility. Now perceive the figure as a duck’s head. Immediately those white streaks cease to be two separate units for our mind. Together with the darker parts surrounding them, they affect our mind as a single unit, the variegated back part of the duck’s head. And at the same time the protrusions to the left make us conscious of hardness, cold, rigidity. The sensory stimulations are exactly the same, but they are differently grouped together, and they bring about further nervous activities which greatly differ in these two perceptions. [Illustration: FIG. 17.--VARIETIES OF PERCEPTION.]