When Alexander turned back at the Hyphasis (Beas), how much did he know about what lay before him? And why, in the vulgate tradition, does he know of the distant Ganges and the distant kingdom of Magadha, but not of the next great river to the Beas, the Sutlej (a question often asked), or of anything else between the Beas and the Ganges? The answer is not difficult, once the elements of our tradition are sorted out chronologically; that, as in so many questions, is the real problem.
We possess one contemporary document bearing on the matter which has escaped notice, a satrapy-list or gazetteer of ‘Asia,’ i. e. Alexander's empire, dating from the last year of his life; very possibly Hieronymus used it by way of introduction to his history of the Successors, and it now forms the basis of Diodorus 18, 5 and 6. We can date this document with certainty. It includes the Indian provinces, and so is later than Alexander's return from India. The ‘Hyrcanian sea’ (not Caspian) is still a lake, so it is earlier than Patrocles. Chandragupta is unknown, so it is certainly earlier than Megasthenes and probably earlier than circ. 302. Porus is still alive, so it is earlier than 317.