Categories are usually fuzzy. That is, when humans use a category, there are usually some members of the category who have all the traits you associate with that category, some members that have many of the traits, and some members where you have to make a judgment call about whether it counts or not.
The Cluster Structure of Thingspace provides several excellent and uncontroversial examples. For instance, think about birds. Robins and sparrows are very typical birds. Eagles are less typical than robins, but still very typical. Penguins are really fucking weird birds. And you have to make a judgment call about bats: for purposes of biology, a bat is not a bird, whereas for purposes of trying to decide which animals are kosher, a bat is a bird. You make the decision based on whether the more important bird trait is “related to dinosaurs” or “flies.”
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