The discussion in this book is related to three categories. The first category comprises things that form the preliminaries of this worship, and' whose identification is necessary for practising this worship. The second category is about things that constitute the elements (arkdn), and these are prescriptions to be acted upon themselves and things that are to be relinquished. The third category is about things that relate to associated matters, and these are the ahkdm for the acts. In fact, each worship can be found to be constituted by these three categories.