Phenomena occur in a context. If the sequence of events comprising a phenomenon are a part of the context, then the question of the reversibility, or lack thereof, of the ordering of events may figure in the phenomenon's explanation. For example, if event A inevitably precedes event B, proffering an explanation might include consideration of whether the dependency inherently takes place in that order. In such situations, causal explanations are often proposed, possibly including the description of a mechanism for producing B from A. The question arises as to how essential considerations of causality are to the explanation process. On the one hand, causality seems to be wired into the human explanatory capacity. On the other hand, there are clearly well-accepted explanations in which causality does not play a role, such as quantum mechanics. Whether or not causality and mechanism are required of an explanation is a function of the explanation's framework.