It has been suggested that a rigid definition for a category is not possible, and that resemblance (much like a family resemblance) might be more appropriate. Why?

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Quizplus · Accepted Answer

Categories and their boundaries are not always clear-cut and there may be borderline cases or items that do not fit neatly into a single category. The family resemblance theory suggests that categories are fluid and flexible, and that membership in a category is based on a cluster of overlapping features, rather than a rigid set of defining features. This allows for greater inclusion of items that may not fit a strict definition of a category.

It Has Been Suggested That a Rigid Definition for a Category