Philosophical activity encompasses an immense subject matter, a content so wide that it is impossible to determine its boundaries with precision. There are texts, considered as philosophical, which on many respects could be viewed as mystical literature, and others which could, quite safely, be classified as scientific. Nevertheless among these innumerable aspects of philosophical activity there are two spheres which belong to the central bulk of philosophical thought: the creation of theories and the criticism of theories. Since the very beginning of philosophy, we find philosophers who have tried to develop theories able to grasp the most profound and general traits of reality; and we have also found philosophers who, in a way or another, have criticized these theories.