Herbal therapy, which in Taoist tradition always includes diet and nutrition, is the oldest and most widely used method in the Chinese system of health care. It evolved over thousands of years by patient empirical observation of nature and trial-and-error experimentation. All Chinese herbs are categorized according to their basic biochemical natures and therapeutic effects, which determine what Chinese herbalists call their gui-jing, or 'natural affinities'. Gui-jing literally means 'return to meridians', and indicates which organ/meridian systems a particular drug has a natural affinity for after it has been ingested and metabolized. There are numerous methods for administering herbal remedies.