According to medieval Aristotelianism, all bodily substances are compounds of substantial form and prime matter. By itself, matter expresses the potency to the realisation of the compound through the reception of a substantial form. Hence, it seems to be completely deprived of any features aside from being a substrate for the reception of forms. Yet prime matter is also the enduring substrate of substantial change, which happens by the acquisition and loss of the forms of the two termini of change. From these two main functions, a plurality of complex issues arose in the Middle Ages touching upon its condition, functionality, and usefulness for the explanation of how the universe works. In my talk, I want to address why, for scholastic philosophers, prime matter was indispensable notwithstanding the many problems arising from its inclusion in their ontology. To do so, I will examine the arguments deployed to demonstrate the existence of prime matter by four scholastic philosophers from the 16^th century: Francisco de Toledo, Pedro da Fonseca, Manuel Góis, and Francisco Suárez. First, I will introduce some main tenets of scholastic hylomorphism. Second, I will analyse five group of arguments used by these authors to assess why they thought they needed prime matter. Finally, I will draw my conclusions.