Hippocrates, On the Epidemics, Book 1, Section 2, Paragraph 5:

With regard to the dangers of these cases, one must always attend to the seasonable concoction of all the evacuations, and to the favorable and critical abscesses. The concoctions indicate a speedy crisis and recovery of health; crude and undigested evacuations, and those which are converted into bad abscesses, indicate either want of crisis, or pains, or prolongation of the disease, or death, or relapses; which of these it is to be must be determined from other circumstances. The physician must be able to tell the antecedents, know the present, and foretell the future--must mediate these things, and have two special objects in view with regard to disease, namely, to do good or to do no harm. The art consists in three things- the disease, the patient, and the physician. The physician is the servant of the art, and the patient must combat the disease along with the physician.