This chapter discusses the radical attack on some tenets of Aristotelian-scholastic philosophy by the sixteenth-century humanist Mario Nizolio. Focusing on his interesting but undervalued work, On the true principles and true way of philosophizing against the pseudo-philosophers (1553, edited by Leibniz in 1670), this chapter shows that his critique involves a radical de-ontologization of the conceptual armory of the scholastics, a turn toward the world of empirical things, a recognition of the central role that the human mind plays in our categorization of the world, and a plea for a clear, transparent language in doing philosophy and communication in general. Rejecting universals as essences as well as Aristotelian categories and transcendentals, Nizolio wanted to defend a horizontal ontology in which concrete things, grouped in classes by a creative act of the human mind, take center stage.Starting with Nizolio’s humanist credo, the chapter then offers an analysis of Nizolio’s criticisms of the traditional five predicables, and his account of the process of abstraction that led to the formation of these universals. After a brief discussion of proof and demonstration, and the relationship between dialectic and rhetoric, the chapter ends by discussing Leibniz’s reaction to Nizolio.