Human assessments of similarity are fundamental to cognition because similarities in the world are revealing.  The world is an orderly enough place that similar objects and events tend to behave similarly.  This fact of the world is not just a fortunate coincidence.  It is because objects are similar that they will tend to behave similarly in most respects.  It is because crocodiles and alligators are similar in their external form, internal biology, behavior, diet, and customary environment that one can often successfully generalize from what one knows of one to the other.  As Quine (1969) observed, “Similarity, is fundamental for learning, knowledge and thought, for only our sense of similarity allows us to order things into kinds so that these can function as stimulus meanings.  Reasonable expectation depends on the similarity of circumstances and on our tendency to expect that similar causes will have similar effects (p. 114).”  Similarity thus plays a crucial role in making predictions because similar things usually behave similarly.

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