 Temporality can be added as a fourth criterion on representations for reductive explanations: (iv) temporal hierarchy: for an abstract hierarchy embedded in an explanation, the entities and their behaviors on the lower level are temporally prior to the entities and their behaviors at the higher levels (Hüttemann and Love 2011). Although temporal or process hierarchies have not received the same attention as spatial hierarchies (such as in Wimsatt’s aggregativity criteria), their importance is seen in explanations that invoke higher level features (e.g., tissue interactions during ontogeny) at an earlier time to causally explain lower level features at a later time (e.g., gene expression patterns). For example, a study of the developmental origin of aortic arch asymmetry involves a mixture of reductionist and non-reductionist representations according to the criterion of temporality (Yashiro et al. 2007). The explanation depicts several events in a sequential process. First, fundamental level properties (gene expression) change non-fundamental level properties (arterial structure), which alters blood flow dynamics (thereby fulfilling the temporality condition). These new dynamics induce a subsequent change in gene expression (thereby violating the temporality condition). This altered gene expression then produces the non-fundamental property of interest, aortic arch asymmetry (thereby fulfilling the temporality condition). Thus, depending on how time is represented and which temporal partitions are utilized, an explanation may be reductive, non-reductive, or both. This is a good example of how ontological and epistemic reduction can yield strikingly different answers. A biologist using a higher level feature at an earlier time to explain a lower level feature at a later time (a non-reductionist explanation by the criterion of temporality) is not denying that this higher level feature is composed of lower level features (e.g., cells and molecules), nor the possibility that the higher level feature was caused by a different set of lower level features at some more distal point in time.