The fact that has caused the greatest uncertainty in regard to the date of Sextus is that Claudius Galen in his works mentions several Sceptics who were also physicians of the Empirical School,[1] and often speaks of Herodotus, supposed to be identical with the teacher of Sextus given by Diogenes Laertius,[2] but makes no reference whatever to Sextus. As Galen's time passes the limit of the second century A.D., we must either infer that Sextus was not the well-known physician that he was stated to be by Pseudo-Galen, and consequently not known to Galen, or that Galen wrote before Sextus became prominent as a Sceptic. This silence on the part of Galen in regard to Sextus increases the doubt, caused by Sextus' own criticism of the Empirical School of medicine, as to his having been an Empiricist. The question is made more complicated, as it is difficult to fix the identity of the Herodotus so often referred to by Galen.[3] As Galen died about 200 A.D. at the age of seventy,[4] we should fix the date of Sextus early in the third century, and that of Diogenes perhaps a little later than the middle, were it not that early in the third century the Stoics began to decline in influence, and could hardly have excited the warmth of animosity displayed by Sextus. We must then suppose that Sextus wrote at the very latter part of the second century, and either that Galen did not know him, or that Galen's books were published before Sextus became prominent either as a physician or as a Sceptic. The fact that he may have been better known as the latter than as the former does not sufficiently account for Galen's silence, as other Sceptics are mentioned by him of less importance than Sextus, and the latter, even if not as great a physician as Pseudo-Galen asserts, was certainly both a Sceptic and a physician, and must have belonged to one of the two medical schools so thoroughly discussed by Galen--either the Empirical or the Methodical. Therefore, if Sextus were a contemporary of Galen, he was so far removed from the circle of Galen's acquaintances as to have made no impression upon him, either as a Sceptic or a physician, a supposition that is very improbable. We must then fix the date of Sextus late in the second century, and conclude that the climax of his public career was reached after Galen had finished those of his writings which are still extant. [1] Zeller, III. 7. [2] Diog. XI. 12, 116. [3] Pappenheim _Lebens. Ver. Sex. Em._ 30. [4] Zeller _Grundriss der Ges. der Phil._ p. 260.