THOMAS AQUINAS (1225 _circ._-1274), was the great Dominican theologian who wrote the _Summa Theologiæ_. In his famous work he incidentally dealt with medical and physiological questions. The source of all motion is the heart. The soul is created anew in each conception. Moisture, heat, and æther alone are necessary for the generation of an individual; the lower animals originate even from putrefying matter. He wrote commentaries on the works of Aristotle, and derived many of his scientific ideas from this great master. The biology of St. Thomas, as may be imagined, is exceedingly feeble, yet it too often forms the only knowledge of the subject which continental clergymen possess. RAYMOND LULLI (1235-1315) was a man of great intellect, who sought the secrets of transmutation of metals and the philosopher’s stone. He was a bold thinker, an astrologer, and a physician of great repute. Naturally he was accused of magic. His acquaintance with the Arabians directed his mind to the study of chemistry. He wrote on medical subjects, the titles of his best known works being _De Pulsibus et Urinis_, _De Medicina Theorica et Practica_, _De Aquis et Oleis_. ROGER BACON (1214-1298). By theologians he was believed to be in league with the devil, because of his belief in astrology and his scientific attainments. It is probable that his reputed invention of certain optical instruments was really due to his acquaintance with Arabic, as the Arabians were familiar with the camera, burning glass, and microscope, which have been attributed to him. Neither is it the fact that he invented gunpowder, as is usually supposed. Bacon wrote voluminously on theology, philosophy, and science. Although he believed in astrology and the philospher’s stone, he had a true scientific idea of the value of experiment, which forcibly reminds us of the Francis Bacon which future ages would reveal. “Experimental science,” he said, “has three great prerogatives over all other sciences: (1) it verifies their conclusions by direct experiments; (2) it discovers truths which they could never reach; (3) it investigates the secrets of nature, and opens to us a knowledge of past and future.”[782] As an instance of his method, Bacon gives an investigation into the phenomena of the rainbow, which is doubtless a very remarkable example of inductive research.