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Full Idea
It has been extremely difficult to find definitions for most natural categories, and even harder to find definitions that are plausible psychological representations that people of all ages would be likely to use.
Gist of Idea
The theoretical and practical definitions for the classical view are very hard to find
Source
Gregory L. Murphy (The Big Book of Concepts [2004], Ch. 2)
Book Ref
Murphy,Gregory L.: 'The Big Book of Concepts' [MIT 2004], p.38