On the Natural FacultiesGalen

Galen wrote On the Natural Faculties in the second century AD as a defense of the Hippocratic and Aristotelian approach to physiology against the rival schools of the Erasistrateans and the Methodists. The work argues that living bodies possess innate faculties (attraction, retention, alteration, expulsion) that cannot be reduced to mechanical processes.

The central polemic is against atomism. Galen insists that the body's capacity to select what nourishes it and reject what harms it requires faculties that act purposively, not mere collisions of particles in void. Digestion is not mechanical grinding; it is a qualitative transformation governed by the organ's natural power. The liver attracts blood, the kidneys attract urine, and each organ exercises a selective capacity that presupposes the teleological structure of the whole organism.

Galen's observational detail is formidable. He draws on extensive dissection (primarily of animals) and clinical experience, and he attacks his opponents not only on theoretical grounds but for their failure to account for what any practicing physician can observe. The work established Galenic physiology as the dominant framework in Western medicine for over a millennium, until Harvey's discovery of the circulation of the blood began to displace it.