 Galileo began his critique of Aristotle in a treatise he drafted around 1590, titled De Motu (On Motion). The first part of this manuscript deals with terrestrial matter and argues that Aristotle’s theory has it wrong. For Aristotle, the matter of the terrestrial realm within the sphere of the moon is of four elemental kinds—earth, water, air, and fire. These possess two formal principles that give rise to their natural motion: heaviness (gravitas; in earth and water) and lightness (levitas; in air and fire). Galileo, using an Archimedean model of floating bodies, and later the balance, argues that there is only one principle of motion—heaviness. Bodies move upward not because they have a natural lightness, he says, but because they are displaced or extruded by other heavier bodies moving downward. So on his view, heaviness is the cause of all natural terrestrial motion.