 This move left Galileo with a problem: what is heaviness and how is it to be described? In De Motu, he argued that the moving arms of a balance could be used as a model for treating all problems of natural motion. In this model, heaviness is the proportionality of the weight of an object on one arm of the balance to the weight of another body on the other arm. In the context of floating bodies, heaviness is the weight of one body minus the weight of the medium. Galileo quickly realized these characterizations were insufficient, and so began to explore how heaviness might be related to specific gravities; i.e., the comparative weights of bodies having equal volume. He was trying to figure out the concept of heaviness that is characteristic of all matter. What he failed to work out—and this was probably the reason why he never published De Motu—was this positive characterization of heaviness. There seemed to be no way to find a standard measure of heaviness that would work across different substances. At this point, he did not have a useful replacement for Aristotelian gravitas.