These passages are marked by the prudence which practical study suggests to a calm and clear-sighted man. They can hardly be said to have opened the way to a Science of Medicine; for in the sense in which we here use the word _Science_, namely, a collection of general truths inferred from facts by successive discoverers, we have even yet no Science of Medicine. The question with regard to the number and nature of the Elements of which bodies are composed began to be agitated, as we have seen, at a very early period of Greek philosophy, and continued long to be regarded as a chief point of physiological doctrine. In Galen's work we have a treatise entitled, _On the Elements according to Hippocrates_; and the writer explains[36] that though Hippocrates has not written any work with the title _On the Elements_, yet that he has in his _Treatise on the Nature of Man_ shown his opinion on that subject. That the doctrine of the Four Elements, Hot, Cold, Moist, Dry, subsisted long in the schools, we have evidence in Galen. He tells us[37] that when he was a student of nineteen years old a teacher urged this lore upon him, and regarded him as very contentious and perverse, because he offered objections to it. His account of the Dialogue between him and the teacher is curious. But in Hippocrates the doctrine of these four elements is replaced, in a great measure, by the doctrine of the Four Humours of which the human body is constituted; namely, Blood, Phlegm, Yellow Bile and Black Bile. Galen dwells with emphasis upon Hippocrates's proof that there must be more than one such element[38]. "What," he asks, "is the method of finding the Elements of bodies? There can, in my opinion, be no other than that which was introduced by Hippocrates; namely, we must inquire whether there be only one element, everywhere the same in kind, or whether there are more than one, various and unlike each other. And if the Element be not one only, but several, various and dissimilar, we must inquire in the second place, how many elements there are, and what, and of what kind they are, and how related in their association. "Now that the First Element is not one only of which both our bodies and those of all other creatures were produced, Hippocrates shows from these considerations. And it is better first to put down his own expressions and then to expound them. 'I assert that if man consisted of one element only he could not fall sick; for there would be nothing which could derange his health, if he were all of one Element.'" The doctrine of One Element did not prevail much after the time of Hippocrates: the doctrine of Four Elements continued, as I have said, long to hold possession of the Schools, but does not appear as an important part of the doctrine of Hippocrates. The doctrine of the Four Humours (Blood, Phlegm, Yellow Bile and Black Bile) is more peculiarly his, and long retained its place as a principle of physiological Science.