An ability to assess similarity lies close to the core of cognition. In the time-honored tradition of legitimizing fields of psychology by citing William James, `This sense of Sameness is the very keel and backbone of our thinking` (James, 1890/1950; p. 459). Similarity plays an indispensable foundational role in theories of cognition. People`s success in problem solving depends on the similarity of previously solved problems to current problems. Categorization depends on the similarity of objects to be categorized to category members. Memory retrieval depends on the similarity of retrieval cues to stored memories. Inductive reasoning is based on the principle that if an event is similar to a previous event, then similar outcomes are predicted. An understanding of these cognitive processes requires that we understand how humans assess similarity. Four major psychological models of similarity are: geometric, featural, alignment-based, and transformational.