[...] the categories that people prefer for natural objects and human artifacts are those that maximize both the similarity between any two members of the same category and the dissimilarity between any two members of different categories. That is, the categories maximize "cue validity": the more that cues or features are associated with the members of a category, and not with the members of other categories at the same level, the better that category is. As for any particular cue, the more it distinguishes the members of the category from the members of other categories, the more VALID it is said to be.
- Clark & Clark (2004), Pag. 156