 The main idea of diachronic reduction is not merely that reduction is a temporal affair but that they consist in the replacement of one theory by another theory, such that one theory (the reducing one) becomes the successor of the reduced theory (Dupré 1993; Rosenberg 2006). It is also often described as a sort of theory-improvement or scientific progress: particularly one in which the laws of the prior theory apply to a proper subset of the cases covered by the laws of the succeeding theory (Dizadji Bahmani et al. 2010).