PREFACE The evidence provided by the literature indicate that the so-called “Silk Road” was not only just the route of commodity circulation, but also the routes of ethnic migration, military conquest, belief transmission, presenting tribute and granting gifts in return, and the route that wealthy traders traveled. Today when we focus on the “Silk Road”, we mainly focus on East-West cultural exchange, and “Silk” here becomes a symbol of cultural carrier. The purpose of study on the “Silk Road” is to search for the lost trajectory of EastWest cultural exchange. To study the “Silk Road”, we are bound to use East-West literatures. The mutual verification of East-West literatures has always been the state that the scholars who research the history of East-West cultural exchange aspire to. Unfortunately, this may only happen fortuitously. The East-West literatures are like the donkey’s lip and the horse’s mouth — if one is slightly careless, one would suffer the ridicule of describing neither donkey or horse. In fact, the EastWest cultural exchange of itself can be regarded as a process of donkey’s lip meeting the horse’s mouth. At least, “neither donkey nor horse”, as intermediate products, is very common. So one might as well laugh and say: the research of the history of East-West cultural exchange is a study of the donkey’s lip and the horse’s mouth.