50. To make this clearer, let us consider the matter in the light of examples. It is impossible for a bird to pursue a course of learning in any science or become trained to any art: it is impossible for a stone to move in any direction, inasmuch as it can only be moved by the motion of another body. Of itself, then, a stone is incapable of moving, and passing from its place. Again, an eagle cannot be taught in the ways of human learning.