premise: When he saw that earlier settlers had built a town on the eastern shore of the Bosphorus, he decided they must have been blind themselves to overlook the advantages of an easily defensible point across the water, and founded Byzantium there, on the site now occupied by Topkap?? Palace.
hypothesis: The settlers did not care that their town was not as defensible as it would have been if it were on the other side of the Bosphorus.