When contemporary writers consider the history of relativism, more often than not, they simply gloss over the entire medieval period, suggesting that the absolute (and absolutely accepted) claims of the Catholic Church during this period blocked the development of any consideration of relativized notions of truth, morals, and ethics. This is an astounding oversight considering the attention that medieval writers themselves paid to the problem in everything from economic treatises to pastoral manuals. Indeed, a comparison between thirteenth- and fourteenth-century commentaries on book IV of Aristotle's Metaphysics (where the Philosopher discusses the opinions of the pre-Socratic thinker Protagoras) reveals the development of proto-relativized epistemologies. The turn towards relativized conceptions of knowledge had everything to do with the possibility of visual error and the recognition that visual evidence is often susceptible to multiple incompatible explanations. This article charts the development of this discovery in the commentaries of Siger of Brabant (ca. 1240-1280s), Thomas Aquinas (ca. 1225-1274), and Nicholas of Autrecourt (ca. 1299-ca. 1369), as well as in the writings of Nicole Oresme (ca. 1323-1382).