'Virtus' and 'species' in the Philosophy of Nature of Roger Bacon (c. 1220-1293)

The paper examines Roger Bacon’s use of the concept virtus in the Communia naturalium and De multiplication specierum. It focuses on the roles which virtus and species play as vehicles of causality in the inanimate realm. It analyses the distinct functions played by virtus in the motion of celestial spheres, the power of natural place, the attraction of iron to magnet, and the universal nature. The analysis concludes that virtus is an efficient power, a feature of form, capable of causing local motion and instigating natural processes. Species is matter’s response to the stimulation made by virtus through which every natural action, to the exclusion of local motion, is made. Species is a non-efficient power, an ‘appetite’ internal to matter. It is an expression of matter’s inherent inclination to promote and perfect itself, the result of matter’s ‘active potentiality’.