In spite of Abu Ma¹shar¹s efforts to situate astrology soundly within a Neo-Platonized Aristotelian natural philosophy, not all medical thinkers of the Arabic tradition bought into his comprehensive scenario, which included judicial, or horoscopic divinatory astrology. Some, following al-Kindi, seem to have accepted his Platonic-Pythagorean preference for numerical harmonies over the data of experience, in which judicial astrology fit reasonably well. Others, beginning in al-Kindi¹s era, with Hunayn ibn Ishaq and Qusta ibn Luqa, at least, and continuing through Ibn Sina and beyond, seem to have accepted astrological influences of a general nature, while remaining skeptical about being able to draw precise prognostic connections between the stars and events on earth. This paper will consider the medical doctrine of the critical days, which straddles empirical science and astrological cosmology, and will examine how various thinkers from the Arabic tradition discussed and classified them. On the basis of this investigation, and as part of a larger project that considers the historical relationship between these two disciplines, conclusions will be drawn, both about the connection of Arabic medicine to its source Graeco-Roman tradition, as well as to its daughter tradition in medieval Latin Europe.