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 antiquity. Yet they constitute a whole, and the different treatises have an organic connection with each other. On the one hand, these works constitute an encyclopaedia, for they contain a résumé and reconstruction of the sciences so far as was possible in the fourth century But on the other hand, they are more than an encyclopædia, because they are a philosophy, in which the universe is explained from the point of view and according to the system of one individual thinker. In them thought and knowledge are mapped out in broad and lucid outlines, with the details sometimes very fully worked in, sometimes barely indicated and left to be supplied by subsequent workers. The key to their arrangement is to be sought from Aristotle himself. From him we learn that science is divided into Practical, Constructive, and Theoretical. Practical science deals with man and human action, and this branch is copiously developed by Aristotle in his ‘Ethics’ and ‘Politics.’ Constructive science treats of art and the laws by which it is to be governed. Towards this branch Aristotle has made but a brief, though valuable, contribution, in his unfinished or mutilated treatise ‘On Poetry.’ Theoretical science has three great subdivisions. Physics, Mathematics, and Theology, otherwise called First Philosophy or Metaphysics. For the section of Mathematics nothing appears done in these remains. Aristotle speaks often of Mathematics as a great and interesting science, capable of affording high mental delight; but he seems to have regarded it as something tolerably finished and settled in his own time, and therefore less requiring his attention than