One of the issues raised in the discussion concerning the nature of cognitive of science of knowledge, especially, in the discussion of the status and classification of knowledge, is the relation of the sciences to the general knowledge contained therein, or to other words how to apply the general knowledge to the subsidiary sciences. In Farabi's view, this discussion is particularly important in relation to the practical philosophy and its underlying knowledge; because unlike other philosophers, Farabi considers practical philosophy not as several sciences but as one science, and wherever he divides practical philosophy into two classes, he refers to these two classes, not as two types of the famous types of practical philosophy that is ethics, domestic politics and politics. The present article focuses on this issue, and after addressing Farabi's statements on the types of knowledge and the type of relationship of knowledge in terms of unity, multiplicity, and how multiple sciences relate to one another, examines Farabi's view of the nature of practical philosophy and its relation to the knowledge of its subsets.