COMMENTARY. The philosophy of sleep is ingeniously treated of by Hippocrates (de Insomniis), and by Aristotle (in his treatise de Somno et Vigilia.) Aristotle states that digestion goes on best during sleep. Pliny’s definition of sleep seems to be taken from Hippocrates: “Est autem somnus nihil aliud quam animi in medium sese recessus.” (H. N. x, 97.) Alexander Aphrodisiensis, in like manner, says, “Know that, during the day, the natural principle is less occupied with its own peculiar operations, I mean the digestion of the food, the changes of the chyle, sanguification, distribution, assimilation, and the like, the mind being engaged in its other energies, namely, the five senses, fantasy, reasoning, and memory; but during night, on the other hand, nature operates more, and the mind less.” (Problem i, 118.) See also particularly Oribasius (Med. Coll. vi, 4); Actuarius (de Diæta, 11); Avicenna (i, 3, 3); Rhases (ad Mansor. iv, 3); Haly Abbas (Theor. v, 35); Alsaharavius (Theor. xi, 3); Averrhoes (Collect. i, 21.) Averrhoes defines sleep to be the recession of the sensorial powers from their organs to the internal parts; and hence, he remarks, those who sleep with their eyes open do not perceive the objects nearest to them. The vital heat being then collected internally, he adds, the powers of the digestive faculty are increased. Rhases agrees with Galen that moderate sleep forms good blood; but that too much corrupts the juices, that is to say, impairs digestion. Haly Abbas remarks that, during sleep, the animal powers are suspended, while the vital and natural continue unaffected; and, therefore, the mind or principle of life being then disengaged, as it were, from one of its offices, is the abler to perform the others aright. Hence, he adds, digestion is best performed during sleep. Alsaharavius approves of taking rest after a meal, but recommends an hour to elapse before going to sleep. The reader will find in Stentzel’s little treatise, entitled ‘Διατριβὴ περι τοῦ ὕπνου,’ an interesting exposition of the opinions of the ancient philosophers and physicians on this subject. There is an ingenious disquisition on the nature of sleep in the commentary of Stephanus on the Prognostics of Hippocrates. In reference to Aristotle’s division of causes, he decides that the efficient cause of sleep is the rest of the particular senses, and of the general sensorium: the material, a humidity which is carried to the roots of the nerves, and prevents the exit of the vital spirit; the formal or organic, the brain, in which the humidity is collected; and the final, or end which sleep serves, the recreation of the powers. (Dietz, T. i, 142.) SECT. XCVIII.—ON WATCHFULNESS.