Riddles of natural categorization

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Abstract
Pigeons and other animals can categorize photographs or drawings as complex as those encountered in ordinary human experience. The fundamental riddle posed by natural categorization is how organisms devoid of language, and presumably also of the associated higher cognitive capacities, can rapidly extract abstract invariances from some (but not all) stimulus classes containing instances so variable that we cannot physically describe either the class rule or the instances, let alone account for the underlying capacity. In contrast, with other contingencies of reinforcement, pigeons will not extract abstract rules of categorization; they will instead learn to identify visual stimuli down to small details, and they will retain much of what they learned for a year and more. How animals can shift between abstraction and photographic retention, and whether or not the two modes can be unified under a single theory are questions that help define the boundaries of knowledge about animal intelligence.