For a long section of Parthian history, particularly the campaign of Corbulo, Tacitus is almost the sole source. His knowledge of eastern affairs was fairly accurate, but his knowledge of eastern geography was not. These facts and his personal biases, which must often be taken into account, make us long for additional checks upon our source. In the long run, however, the picture which we secure is probably substantially correct.
Since the internal condition of Parthia at the time when Vardanes had been ruling "two years and eight months," as described in Philostratus' Life of Apollonius of Tyana, fits the facts as far as we know them, it seems unfair to condemn this whole work as unhistorical. These very figures speak for the accuracy of some details, since they are numbers not usually utilized in fictitious accounts. But the question of how much may be employed with safety remains a difficult one. Of a somewhat similar nature are the Sibylline Oracles, which among much else contain a certain amount of historical data. When, however, it is impossible to tell with any certainty to what individuals or events or period the information pertains, speculation along these lines would seem of little real value. The best one can do is to point out the existence of the material and its most probable place in the general narrative. Certain books seem a unity, free from much or any later interpolation, and these prove extremely useful.
