 Another minimal criterion of adequacy is generally taken to be that the highest categories (or, for tree systems, the categories at each level of branching) be mutually exclusive, ensuring that whatever there is (or might be) finds its place in exactly one highest category, or one category at each level (Chisholm 1989, 162). (This still allows for nested categories, so that something may belong to both a more specific category like substance and a more general category like individual.)