  Zabarella recognizes two different ways in which a physician can know the parts of a human body. First, he may learn them through perceptive knowledge and anatomical observations, thereby assimilating the matter of his discipline without understanding its rationale. A physician can also become familiar with the parts of human body through philosophy of nature where he may learn the reasons, which lie behind what he actually sees. Zabarella thinks Aristotle made the same distinction in his books the History of Animals and the Parts of Animals. In the first he relies on sense perception to classify the different parts of animals. In the second, he offers causal explanations for what he is considering. In Zabarella’s opinion this order of understanding results from our own inability to comprehend everything at once. It is thus better to progress gradually from confuse to distinct knowledge. In De rebus naturalibus Zabarella points out that the art of medicine adopts the physiological part from the philosophy of nature. If medical writers want to know the anatomy of the human body, they must therefore follow Aristotle methodologically. Therefore they should not study the History of Animals, but instead the Parts of Animals, which shows us the functions of different parts of the bodies in question. The subject-matter of medicine involves maintaining or recovering health only in human beings, not in other animals. Since the whole discipline deals only with the human body, it cannot be a science in Zabarella’s view. What a natural philosopher writes about animals, a medical writer should apply to human beings.