The so-called "argument from illusion," well known from contemporary discussions of the nature of perception, makes a notable appearance in the fourteenth century in Peter Auriol's discussion of apparent or objective being. Rather than establishing the existence of sense data, the argument is employed by Auriol to show that in every act of perception the object perceived exists in apparent being. The goal of this contribution is to explore Auriol's version of the argument. After an analysis of Auriol's argument, I turn to the extensive criticism that William Ockham dedicates to the argument in his Ordinatio. I try to show that Ockham's attack on Auriol is based on a misunderstanding: Auriol is not using the argument to posit intermediary items in the process of perception, nor is his reasoning obviously fallacious. Seeing how Auriol can avoid Ockham's criticism allows us to better appreciate the nature of Auriol's teaching on objective or apparent being and to comprehend the real function of the argument.