Are rules constructs of linguists or descriptions of actual associations latent in the speakers' mind? An interesting method of verifying the psychological reality of rules has been proposed by P. Kiparsky (1968). It can be summed up as follows. Given two chronological stages of a language (or two languages) where one can be said to be descended from the other, one first has to describe a phenomenon in the ancestor-language, then seek out and describe the phenomenon that can be considered a development thereof in the daughter-language. The two phenomena may be found somewhat different in certain respects, though still similar enough to be recognized as historically related. If the surface difference between the two phenomena, conceived as a change from the first to the second, can be explained, by comparing the formulation of the two descriptions, as a plausible modification of the first (set of) rule(s) naturally leading to the second, the chances are that the rules actually describe what appears in the speakers' mind, as part of collective knowledge. For Kiparsky, such a plausible change is primarily simplification.