 One branch of ancient skepticism had its roots in Plato’s Academy, which from the time of Arcesilaus’ headship in the mid-260s BCE turned in a skeptical direction, arguing that there was no clear criterion of truth and so philosophers can only suspend judgment. While that was an “Academic” position within philosophical debate, a possible forebear (Pyrrho) had in the fourth century BCE putatively demonstrated the tranquility of mind which such a position could bring, by living a life seemingly without any investment in the truth of his beliefs (though the question of how, and how far, Pyrrho was guided by appearances would perplex later followers). (See the entry on  Pyrrho.)  From the first century BCE, a radical movement of “Pyrrhonist” skepticism was developed outside the Academy by Aenesidemus, followed about a century later by Agrippa, in a movement summed up for posterity in the second-century CE work of Sextus Empiricius (Outlines of Pyrrhonism) which would have a considerable influence on early modern philosophy. Meanwhile the milder “Academic” skepticism, allowing the inquirer to incline to one side or another though not to invest in full belief, had significant influence on first-century BCE figures such as Cicero. (See the entry on  ancient skepticism.)