 One may have to deny that metals form a natural kind, and that classification by functional group is a classification into natural kinds. (Not all classifications need to be into kinds in order for them to be useful.) Note that in both cases, there is room for vagueness. Some elements, such as germanium and antimony, are classed as metalloids, with characteristics between metals and non-metals. The impact of a functional group diminishes with the size of the molecules of which it is a part and in the presence of competing functional groups, and so in certain cases, classification according to functional group will be vague. Such vagueness falls foul of the categorical distinctness requirement (Criterion 6 above).