 The individual substances are the subjects of properties in the various other categories, and they can gain and lose such properties whilst themselves enduring. There is an important distinction pointed out by Aristotle between individual objects and kinds of individual objects. Thus, for some purposes, discussion of substance is a discussion about individuals, and for other purposes it is a discussion about universal concepts that designate specific kinds of such individuals. In the Categories, this distinction is marked by the terms ‘primary substance’ and ‘secondary substance’. Thus Fido the dog is a primary substance—an individual—but dog or doghood is the secondary substance or substantial kind. Each arm of this distinction raises different issues. If one is concerned with kinds of substance, one obvious question that will arise is ‘what makes something a thing of that kind (for example, what is involved in being a dog)?’ This is the question of the essence of substantial kinds. But if one is concerned with individuals, the parallel question is ‘what makes something that particular individual of a given kind (for example, what is involved in a dog’s being and remaining Fido)?’ This is the question of individual essences and of identity over time. Aristotle was mainly, if not exclusively, concerned with questions of the first kind, but, as we shall see in  section 2.5.2  and  section 3,  the latter question assumed a prominence later.