DEFINITION AND ROLE OF SIMILARITY CONCEPTS IN THE CHEMICAL AND PHYSICAL SCIENCES

After an introductory discussion of the ubiquitous yet elusive concept of similarity and its role in the physical sciences, we define in mathematical terms the four major kinds of similarity that have been exploited in the scientific domain to date. The similarities in question are analogies, complementarities, and equivalence and scaling relationships. Each is defined in set-theoretical terms with crisp sets, this being possible because the emotive content of the entities and events typically studied in the physical sciences is effectively zero. Illustrations of the use of each kind of similarity in the chemical context are given, and reference is also made to the increasing use of similarity methods in the optimal design of molecular structures for specific applications.