  In the last chapter of De naturalis scientiae constitutione Zabarella discusses the question of the perfection of the natural sciences (De perfectione scientiae naturalis ac de eius ordine). Zabarella states that Aristotle’s philosophy of nature may be perfect in structure and form, but it is incomplete in terms of its reference to natural beings. There is much Aristotle did not discuss at all and indeed much that was outside his cognisance. Although he dealt comparatively little with plants and animals, it is not difficult to pinpoint their proper palces in the Aristotelian system of the natural sciences. Therefore Zabarella emphasizes that Aristotle’s philosophy of nature is complete at least in theory. Zabarella compares Aristotle’s works on natural philosophy to the geometry and arithmetic of Euclid. There are many theorems which can be demonstrated from his works even if he did not himself actually write them. For Zabarella this is no reason to judge Euclid’s geometry or arithmetic defective or incomplete. If Euclid had wished, he could have demonstrated all the particular cases, but his book would have become so enormous that it surely would have daunted the reader. Zabarella suggests that this is exactly why Euclid entitled his book The Elements, and from this foundation all the other theorems can be demonstrated.