"A physician becomes perfect in the waves of change. Adherence to rigid principles will only create stagnation. Every wanderer on the path of medicine necessarily looks to the laws of nature expounded by the masters of old, but when the moment of action arrives, s/he must receive the transmission of the heart and become enlightened to the atmosphere of the moment when the sages first recognized these laws creating these laws oneself, over and over again."
"The upper class of medicines.....govern the nourishment of destiny and correspond to heaven...., If one wishes to prolong the years of life without aging, one should" use these.
"The middle class of medicines govern the nourishment of one's nature and correspond to man. ...If one wishes to prevent illness and to supplement depletions and emaciations, one should" use these.
"The lower (class of) medicines....govern the treatment of illness and correspond to earth. If one wishes to remove cold, heat and (other) evil influences (from the body), to break accumulations, and to cure illnesses, one should base (one's efforts) on (drugs listed in) the lower (class of this) manual."
Shen Nong Ben Cao