William of Ockham was tried for heresy due to his assertion that certain qualities can exist independently of substances. Scholars have assumed he made thisstrange assertion in order to account for the Catholic doctrine of transubstantiation. I argue, however, that the assertion was philosophically rather than theologically motivated. Ockham develops a nominalist substance ontology, according to which most changes can be explained as the result of local motion. Knowledge and virtue are changes in human beings that cannot be so explained, however, because they are not entirely passive processes. In fact, knowledge and virtue require free will, which could not be considered truly free if it were not an independently existing quality. In this paper, I explain Ockham’s nominalist substance ontology and show how it functions as the sine qua non foundation for his uncompromising commitment to metaphysical libertarianism.