Deeply challenging, educationally critical discipline content often resides in the realm of the existentially unfamiliar, that which is initially experienced as unknown--and unknowable. By definition, student encounters with this content lead to a rupture in knowing, and initiate an interval of confusion, which in turn engenders uncertainty and unease. Only by taking on uncertainty can one productively navigate the confusion in order to learn, and expand knowing to encompass the previously unknowable. Turning away from the uncertainty and unease disallows learning. The former is known as reflectiveness, the latter as defensiveness. This paper is written as part of an ongoing exploration into student learning in computing. The ongoing exploration is motivated by questions of how to engender a reflective response (which is required for transformative learning) among all students; or, among those students who do respond defensively, how to cultivate transition to reflectiveness. The exploration defines and sets context for the individual investigations; each investigation contributes to and refines the overarching conceptual framework of the ongoing exploration. This paper was undertaken in order to refine the overarching framework; it is intended to further clarify the conceptual infrastructure regarding defensiveness. In the process, the value of explanatory theory is confirmed for computing--and other--education.