Heaviness, Lightness and Impetus in the Seventeenth Century: A Jesuit Perspective

This paper examines the mechanical project of a Jesuit scientist-engineer Paolo Casati (1617–1707), as presented in his Mechanicorum libri octo (1684). The core of the project consists in an attempt to physicalize the mathematical science of machines while using Archimedes’s lever principle and his theory of buoyancy. Casati, however, aimed at enriching this science with principles originating in the Aristotelian science of motion. The fusion of Archimedean and Aristotelian elements—including the concept of impetus as the sole cause of motion—allowed him to deal with the motion of machines in terms of a hydrostatic theory of motion, neutralizing the Aristotelian notion of “positive lightness,” but without erasing it. This he did while engaging himself with experiments on “positive lightness” done at the Academia del Cimento (1657–1667). While this analysis throws light on the transformation of Aristotelian science in Jesuit schools of the seventeenth century, it also frees some Jesuits, at least, from a far too simplistic historiographical category of Galileo’s “nemici” (foes) and inserts them instead into the field of debates relevant for understanding the emergence of “the new science.”