A pony in a circus act might be trained to respond in various ways to colored disks and squares. Only a human being, however, can derive from these various circles and squares the definition of a circle and of a square. A person can also know by abstraction the properties of these figures, such as the relation of the circumferences of a circle to its radius, which he expresses in the abstract formula C = 2 R. Such abstract knowledge is clearer although it is less vivid than the sense apprehension of the colored figures, which the pony can share with a human being.