Agrippa is known by way of one citation in Diogenes Laertius's Lives of the Philosophers (DL 9.88). Nothing is known of his life, and little of his dates (he lived between the mid-first century BCE and the second century CE). Yet Agrippa is indisputably a figure of the highest importance in the history of skepticism, indeed of epistemology in general. The citation attributes to him the invention (or at least the codification) of five "Modes," or argument patterns, which represent a new methodological rigor and self-consciousness in the development of Pyrrhonian skepticism. Earlier skeptics such as Aenesidemus had presented certain aspects of skeptical procedure in a more or less organized fashion; but the Ten Modes attributed to him are arranged according to the subject matter of the considerations appealed to. By contrast, the Modes of Agrippa seek to categorize...