"Oral tradition is always something spoken." So begins the first line of Eric Eve's Behind the Gospels: Understanding the Oral Tradition. With this statement, we must recognize an obvious heuristic reality: by definition, nothing that we read in ancient texts is oral tradition. There is no oral tradition in the gospels, by definition. How then can scholars claim to study the oral tradition "behind" the gospels?
The answer is obvious: inference. This is exactly how historical investigation always works. By definition, we do not find the past in the material that we utilize in the work of historiography. The texts, the artifacts, the architecture, etc.: these remains of the past all exist in the present. None contain the objects that historians want to know about, the entities, relationships, events, and processes that we seek to define and understand. Yet, we can use such remains of the past to infer such matters. Oral tradition is just one such an entity that we seek to define and understand, oral traditioning one such process.
This raises a critical challenge to the current historical skepticism that grips much of NT scholarship, and has in various forms for much of the last century. We are told confidently that we cannot know very much about what the early Christians were doing in the first decades of the movement. Why? Because we do not have direct access to that time. Our knowledge of the period is mediated by texts, by artifacts, by architecture, and such mediation constitutes a barrier to knowledge. Yet, the same skeptics frequently have no difficulty telling us how oral tradition processes worked in that same time, using the same remains of the past as their primary data, using the same basic practice of inference that is used to define and understand any other objects from that time. Indeed, their skepticism is typically predicated upon their understanding of how these remains came about: precisely because they judge these remains to be the result of complicated processes, they judge them to be of limited utility for the work of history. Except: that can only be argued if these remains have already been used in the work of history! Put otherwise, such skeptics are denying the very conditions upon which their skepticism is predicated.
If we can know that "behind" the gospels lies oral tradition, then there is no reason in principle that we can't also know, for instance, who was involved in developing the oral tradition, or where such development took place, or when. If we can answer "What?", then "Who?", "Where?", "When?", even "Why?" are in principle also answerable. And if we can know these things, then there is no reason in principle that we can't know how these matters interact with other objects from this period: with the expansion of the Christian movement, the development of ecclesiastical structures, the elaboration of doctrine, etc. If we can infer one thing about the past from texts and artifacts and other remains in the present, then we can in principle anything. "Postmodern" arguments about the impossibility in principle of knowing about the past dissolve immediately, corroded by the acid of self-reversal. There can of course remain arguments that the extant remains of the past are such that we cannot answer this or that particular question in practice, but arguments from principled skepticism are refuted by the very act of making statements about what happened in the past.