The Complete Works of Claudius Galen, Vol. 18 Pt. 2

[583] ...as is fitting, certain physicians of his time, such as those who even now call themselves Methodists, who claim that the work of a physician is either to preserve existing health, as in the case of those who are well, or to restore it when it has been corrupted, as in the case of the sick, while the work of a prophet is to foretell what will come to pass. Hippocrates, therefore, demonstrates through this preface that it is useful for a physician to possess foresight, bringing his argument into three chapters: first, that he will have the sick more obedient to his prescriptions; second, that by having foreseen what will happen to the patients, he will have prepared himself for those events long in advance; and third, that he will remain blameless for the death of his patients. Nevertheless, he desires that this too should be present for him: that he also be considered the cause of their health. Let us now, therefore, attend to the passages from which he establishes each of the aforementioned points.

...certain physicians, as is fitting, who existed in his age, just as there are those now who call themselves Methodists, and who assert that the office of the physician is either to protect present health, as for those who are well, or to restore it when lost, as for those who are sick, whereas it is the office of a prophet to predict the future. Therefore, Hippocrates demonstrates by this preface that prognostication is most useful to a physician, the argument being led into three principal causes: first, that he will have the sick more obedient to his commands; second, that having foreseen the events that will happen to the patients, he will have prepared himself against them long before; third, that he will not be judged the author of the patients' deaths. Moreover, he wishes that it be no less the case for him that he be esteemed the cause of their health. Let us now, therefore, apply our minds to the words by which he establishes those principal points.