Organon § 120

Medicines must be thoroughly tested on the healthy to ascertain each medicine's powers and true actions and to differentiate the medicines from one another. 

Therefore the medicines upon which the life and death, the disease and health of human beings depend must be exactly, painstakingly distinguished from one another. For this reason, through careful, pure experiments, they must be tested as to their powers and true actions in the healthy body so that we may become exactly acquainted with them and avoid every mistake in their use with diseases, since only an apt selection of medicine can quickly and permanently restore the greatest of earthly blessings, the well-being of body and soul.