 Truth is one of Protagoras’ most famous books. Ancient books were not necessarily given their titles by their authors; but Plato and Aristotle continuously refer to Protagoras’ text as Aletheia and also make puns, confirming that Protagoras himself gave this title. The notion of “truth” played a prominent role among the Presocratics: in choosing this title, Protagoras clearly wanted to present himself as an heir to this long-established tradition; and by questioning the existence of one single truth, as we will see, he sought to challenge it. The first sentence of the treatise fully confirms its author’s provocative intentions: “Of all things the measure is man: of those that are, that they are; and of those that are not, that they are not” (80B1 DK; for the sake of convenience, this sentence will be abbreviated as MM). MM is one of the few authentic Protagoras fragments to have been preserved and many ancient sources confirm that the statement was originally formulated precisely in these terms. The problem, however, is its opacity: ancient readers debated what Protagoras really meant, and modern interpretations have only increased doubt. As a matter of fact, it cannot be ruled out that a certain degree of ambiguity was intentional. Truth was not a scientific treatise but an epideixis, the text of public performance, which was meant “to demonstrate one’s virtuoso skill in argumentation” (Lee 2005: 27, after Lloyd 1987: 96). In opening his speech, Protagoras sought to capture the attention of his audience (or reader) by making a striking and allusive claim. There is nothing odd in all of this, since a desire to capture the audience’s attention was quite typical of the sophists, and is confirmed by many other sources (the sentence introducing the treatise on the gods was just as provocative, see  §4).