To reduce its power: let the medicine be compounded; for when it is too hot, too sour, another medicine is mixed with it which reduces its heat and its piquancy, as is obvious in the case of gum arabic which is mixed with theriac.
To preserve the virtue of the medicine so that it might not become putrid: let the medicine be compounded for it is necessary for the physician to keep a time record of the medicines over a long period of time. For this reason, so that their virtues might not depart, it is necessary to mix another item with them to preserve their virtues, as is obvious from opium which is mixed with other composite medicines.