  Structural kinds apply to categorial individuals and kinds on the basis of other, ultimately categorial properties those entities have. For instance, two objects are equal or unequal if they have some definite size or duration, or if they are numerable. Two objects are like, if they have one or more properties in common. A red cube and a red ball are like because they share redness in common.  They are unlike, because they have different shapes. The nature of a structural kind is determined by its functional role in enabling categorial kinds to be what they are and/or to associate with or differ from one another.