Goldstone, R. L., & Day, S. B. (2013).  Similarity. In H. Pashler (Ed.) The Encyclopedia of Mind. SAGE Reference: Thousand Oaks, CA. (pp. 696-699)

An enormous amount of ink has been spilled in the psychology literature on the topic of similarity.  There are two reasons that this seemingly intuitive and prosaic concept has been the subject of such intense scrutiny.  First, there is virtually no area of cognitive processing in which similarity does not seem to play a role.  William James observed that “This sense of Sameness is the very keel and backbone of our thinking” (James 1890/1950: 459).  Ivan Pavlov first noted that dogs would generalize their learned salivation response to new sounds as a function of their similarity to the original tone, and this pattern of generalization appears to be ubiquitous across species and stimuli.  People group things together based on their similarity, both during visual processing and categorization.  Research suggests that memories are retrieved when they involve similar features or similar processing to a current situation.  Problem solutions are likely to be retrieved from similar prior problems, inductive inference is largely based on the similarity between the known and unknown cases, and the list goes on and on.
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