It is a familiar and pervasive feature of thought and speech that often when we are able to refer to many distinct entities, x ₁, x ₂, ... , x _n, we also seem able to refer to an entity y which in some loose sense we take to include or contain x ₁, x ₂, ... , x _n. Let us say that in such cases y comprises x ₁, x ₂, ... , x _n, and so describe y as a comprising entity. A tree, we may say then, comprises its cells. The set (in the ordinary, non-mathematical sense) whose members are the tree's cells may also be said to comprise those cells. We usually consider the relation between the cells and the tree to be compose. The relation between the cells and the set of cells we normally take to be a different relation, are all the members of. However, these relations have general features in common which seem to justify our taking them as species of one and the same generic relation.