“For just as all the sophisticated doctors and most sophisticated athletic trainers pretty much agree that those who are to be good doctors or trainers must be experienced about nature – and indeed much more than the former … in the same way, the statesman must have certain norms taken from nature itself, i.e., from the truth, by reference to which to judge what is just and what is good and what is advantageous. For just as in building these tools surpass all, so too the finest law is the one that has been laid down most in accordance with nature. But this is not something which can be done by someone who hadn’t done philosophy and become familiar with the truth.” (Ps-?)Aristotle’s Protrepticus
Aristotle’s natural treatises offer various versions of a scala naturae, all of which place human beings at the top: humans have the ‘most perfected [material] nature’ of all living beings and possess the greatest share in living well and in the divine (PA II 10, 655b37-656a8):
Since, then, it is the nature of plants to be immobile, their non-uniform parts are not of many forms: for there is use of few instruments for few activities. Therefore we must investigate their forms separately. Those beings which have perception in addition to life are more polymorphic in their form, and of these some more than others, and there is still greater variety among those whose nature partakes not only of living but also of living well (ὅσων μὴ μόνον τοῦ ζῆν ἀλλὰ καὶ τοῦ εὖ ζῆν ἡ φύσις μετείληφεν). And such is humankind; for he alone, or he most of all, partakes of the divine (μετέχει τοῦ θείου) among living beings known to us.
As is clear also from this passage, Aristotle distinguishes in the biological treatises between two kinds of functions that may be served by the various soul-capacities any kind of living being has: some parts and capacities are necessary for the sake of living, i.e., for basic survival and reproduction, whereas others allow animals to live well, where living well is understood as displaying a greater organic diversity and functional complexity than is required for ‘mere’ survival. Only humans possess the additional capacity for thinking, which according to Aristotle allows them to partake in the divine, as thinking is essentially a function of the divine (see PA IV 10, 686a26-32 and GA II 3, 736b27-29).
Once we turn to the ethical treatises, however, Aristotle makes clear that the kind of living well he has in mind for human beings comprises more than just displaying the greatest functional complexity of all living beings: the best realization of a human life is the life of virtue and contemplation, i.e., the life of happiness. For instance, in the Nicomachaen Ethics, Aristotle argues that the kind of capacity for thinking that is crucial for living a happy life is not one that humans share with animals and that exercising our ‘divine function’ therefore gives rise to a radically different kind of living, one that focuses not on the biologically necessary or pleasant, but on the noble and the morally good (see, e.g., EE I 7, 1217a21-29; NE I 7, 1098a7-18; NE I 9, 1099b33-1100a4; NE VI 5, 1140a24-28; and NE X 7, 1177a12-18). Similarly, in the Politics, Aristotle emphasizes this distinction between the biological and the ethical in his account of the city: it might well be that cities come to be naturally for the sake of biological survival or ‘living’, but that is not their true purpose, otherwise ‘slaves and animals could have a city’ (Pol III 9, 1280a31-34). Instead, cities are present for the sake of living well, which Aristotle equates with self-sufficiency, happiness, and the good (Pol I 2, 1252b27-30; Pol III 6, 1278b15-31; Pol III 9, 1280b39-1281a4).
In this way, the biological and the ethical realm are separate for Aristotle, and this seems to be even more true for the two sciences that pertain to these realms. For the objective of political science – even though Aristotle sometimes qualifies it as a ‘philosophy concerning human matters’ (EN X 9, 1181b15: ἡ περὶ τὰ ἀνθρώπεια φιλοσοφία) – is not to generate knowledge about humans for its own sake, but finding the best ways to make humans – and in particular, freeborn male citizens – ‘good and capable of fine deeds’ (see, e.g., EN I 9, 1099b29-32, EN I 13, 1102a7-12, EN II 1, 1103b2-6 and Pol VII 13, 1332a7-38). Political science is a practical discipline (EN II 2, 1103b26-31; cf. EN II 4, 1105b12-18), not a theoretical one, as natural science is (cf. Meta VI 1 and XI 7).
For these reasons, scholars of Aristotle sometimes assume that the ethical treatises can be studied autonomously – that neither Aristotle’s intended audience nor the contemporary reader needs any specialized kind of knowledge about the natural world to understand Aristotle’s views about what is good for a human being and how to become such. In other words, while it is generally acknowledged that Aristotle’s political science was not developed in a philosophical vacuum and that it therefore presupposes, among other things, a certain kind of metaphysics (involving forms and capacities that need to be realized), philosophy of nature (involving a theory of natural teleology), and psychology (involving a division of the soul in at least a rational and an irrational part), scholars have not always sufficiently appreciated how much Aristotle believes knowledge of such fields is relevant to the person who wants to become good, and in particular, to those who are studying to become future lawgivers and want to make others good, who I believe form Aristotle’s primary audience.
In the paper that I am currently working on, I hope to show that if someone wishes to become a successful lawgiver of a truly happy city on the basis of studying Aristotle’s ethical treatises, he better passes his Biology 101 class first.
 From Iamblichus, Protrepticus X 54.12-55.3; translation by Hutchinson & Johnson 2005, 263.
 Thus e.g. Kraut (2010), in his entry on Aristotle’s ethics in the Stanford Encyclopedia of Philosophy, section 3.2: “Even though Aristotle’s ethical theory sometimes relies on philosophical distinctions that are more fully developed in his other works, he never proposes that students of ethics need to engage in a specialized study of the natural world, or mathematics, or eternal and changing objects. His project is to make ethics an autonomous field, and to show why a full understanding of what is good does not require expertise in any other field.” (My italics.)