According to Aristotle, substance is form, which in general can only exist when combined with matter. But material things have all kinds of different attributes contributing to their forms. Dogs, for example, have different breeds, colors, sizes but are all still dogs. On Aristotle's view, some of a dog's attributes make up its substance and others do not. Color and size, for example, are attributes that are not in the category of substance whereas "animal," "mammal" and "dog" are. We may note that these concepts can be organized hierarchically from general to specific. In the case of living things, this hierarchical organization forms the basis of the classification scheme still used in biology.