Identifying the drug-producing plants, then, was not the only factor in early pharmaceutical and medical practice. It was one thing to be able to recognize a drug plant, even to know it's popular name; it was another to know how to extract and purify the active ingredient, and, above all, to know the right dosage. There were other complications. Some drugs were so powerful that they could only be safely administered on certain days, or after lengthy preparation of the body and mind. It was also well known that over-powerful drugs had to be countered with another having the opposite effect, as in the case of the purge Hellebore, and with some narcotics which had to be offset with stimulants. To know the correct dosages in these cases required an appreciation of the susceptibility of the patient to the drug's effects, perhaps the most difficult calculation of all. Much depended on the recipient's 'fate' allotted him at his birth, the factor that determined his individuality, his physical stature, the colour of his eyes, and so on. Only the astrologer could tell this, so that the art of medicine was itself dependent for success on astrology and the considerable astronomical knowledge this presupposed.

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