This chapter studies Lorenzo Valla’s critique of the language and thought of the scholastics. Valla contrasts classical Latin, as a natural, common language, with the so-called artificial, technical, and unnatural language of his opponents. He famously champions Quintilian’s view that one should follow common linguistic usage. Scholars, however, have disagreed about the precise interpretation of these qualifications of Latin. This chapter argues that, depending on the historical, rhetorical, and argumentative contexts, Valla uses notions such as common and natural in different ways to suit different purposes. After an examination of Valla’s notion of common linguistic usage, which is shown to refer mainly (though not always) to classical Latin, the chapter analyzes Valla’s Dialectical Disputations, which attacks metaphysical and logical concepts from Aristotelian philosophy. It is shown that Valla combines two levels of criticism: scholastic Latin is ungrammatical and it is unnatural. To make both points Valla moves between a strictly Latinate point of view, and a sociological perspective according to which use of language should follow the rules and conventions of the community. The chapter argues that this leads to a tension in Valla’s humanist project concerning Latin as a common language and the notion of the common people.