The author examines the question of Galen's affinity with astrology, in view of Galen's extended astrological discussion in the De diebus decretoriis (Critical Days). The critical passages from Galen are examined, and shown to be superficial in understanding. The author performs a lexical sounding of Galen's corpus, using key terms with astrological valences drawn from the Critical Days, and assesses their absence in Galen's other works. He compares Galen's astrology with the astrology of Ptolemy's Tetrabiblos, and evaluates their respective strategies of scientific reasoning. Three types of inference are introduced and applied to Galen's astrology. Finally, he concludes that the empirical side of Galen's science does not depend upon astrological methods or concepts, but that these were introduced for their rhetorical effect in presenting his new medical methodology. It is suggested that continued attention to Galen's astrology has obscured the truly important empirical scientific method that Galen developed.