Editing Genidentity (section)

== Classification of Natural Sciences ==

In his thesis mentioned above, Lewin compares [[physics]] (in which he includes [[chemistry]]) and [[biology]] (which he divides into organic biology and [[evolutionary history of life]]). A comparison of this kind presupposes that it is possible to find equivalent notions in both sciences. According to Lewin, the concept of ''genidentity'' in the various sciences meets this requirement.

Lewin distinguishes between partial and total genidentity. This is due to the difficulty caused by parts of objects: for instance, an object might disintegrate into several pieces in the course of its development. As we follow such an object through time, only a small portion of it may remain. Lewin says that two objects existing at different times are partly genidentical if at least some part of the later object was present in the earlier object. By contrast, he says that two objects are totally genidentical if and only if at neither of the considered times there is any distinct object partially genidentical to one of the two objects concerned. 

Lewin also introduces the idea of viewing physical bodies as links in a so-called ''development chain''. According to this approach, between two totally genidentical objects there always exists, at any intermediate time, an object totally genidentical with each. Thus genidentity implies the existence of an entire infinite series of intermediate objects. In this Lewin sees an analogy between physical objects and [[real number]]s, as defined by so-called [[Dedekind cut]]s in the ordering of [[rational number]]s. 

Genidentity so defined is postulated to have various characteristics, such as symmetry, transitivity, density, and continuity. When reviewed in the light of contemporary standards of logical precision, it becomes clear that Lewin had the correct intuition, even though he did not have the benefit of a highly developed terminology of definition theory or modern day [[symbolic logic]]. 

However, genidentity has never been explicitly discussed in the experimental sciences. Rather, it has always been a basic assumption hovering in the background, tacitly assumed. The credit for having made these assumptions explicit for the first time doubtlessly belongs to Kurt Lewin, usually renowned for his psychological work in the field of [[Gestalt psychology]].