Astrological calculations were thought absolutely necessary to determine the appropriate time to commence or change treatment and planetary movements were regarded as strongly influencing the patient's prognosis.

Since disease was considered by Galen and Hippocrates to be a divergence from the natural healthy state, and since the stars ruled all change in sublunary nature it was the stars which were considered to be the causes of health and disease. Since they were viewed as spiritual forces, there was an implicit assertion of an intimate relationship between body and soul: the health of one depended upon the harmony and well being of the other. 

The astrological causes could disturb or harmonize either. For the Medieval astrological physician, God had placed the luminaries (being the Sun and the Moon), the planets and stars in the immutable quintessence of heaven pursuing immutable cycles so that man could, in accordance with Genesis 1:14, know the
"signs" and "times" (mentioned in Ecclesiastes) for such things as the treatment of disease.