The basic concepts found in medieval philosophy relative to the characterization of the sphere of theory and that of practice were derived from Greek philosophy, especially from Aristotle. According to these streams of thought, theory is concerned with the cognition of reality, the practical act with directing the will toward that which it considers proper, while the fundamental principle of the poetic act is to give form to the sensible or material sphere. These observations of the attempts of medieval philosophy to clarify the concepts dealt with here are not made merely for the sake of completing the historical development of these concepts but rather to gain some insight into their nature. Even some of the blurred distinctions of the medieval philosophers, especially in the sphere of the act, will prove to be of value for the proper understanding of the concepts current in our day.