According to its form, the rational animal is not for the sake of any other species at all--not by way of material, nor by way of being an instrument or of serving. Every of them beneath it, by right of its form, is either only for something else or brings both of them together: existing for itself and existing for something else. Justice is for both its portions to be accorded by nature.--Al-Farabi, Political Regime, 54 (in Butterworth's translation, p.55).
Al-Farabi's Political Regime, which is not very long, can be divided in roughly four parts: (i) the longest part: a highly technical and compressed metaphysics (and cosmology/modal logic); (ii) a brief causal account of political variety; (iii) a serious description of the best/virtuous city that is longer that Socrates's city of pigs, but considerably shorter than the luxurious city; (iv) a quick survey the many kinds of non-virtuous cities.
Part (i) is not accessible to those that lack grounding in Platonic and Aristotelian metaphysics/terminology. So, unlike Plato's Republic, which draws the reader into the topic with sharply drawn and exciting exchanges in the first two books, Al-Farabi is clearly writing for a rather limited audience. This is no surprise because Al-Farabi is an elitist about cognitive ability and thinks that even among those with high cognitive potential few cultivate it.One might then think, perhaps alerted by Leo Strauss's legacy on the reception of Al-Farabi, that the first very technical part is the intellectual barrier and smoke-screen for the political teachings that come at the end or, more subtly, are woven into the technical bits (about which below).
Even if the thoughts of the previous paragraph are not incorrect, it does not follow that the intended audience for this book is (only) philosophers or would-be philosophers because the book as a whole does not develop many of the points it introduces. In fact, it treats many issues in a kind of overview, summary manner. And while it is hard-going at first, it quickly rewards the reader who is interested in politics and ruling. So, the book seems to pitched not so much for those with extended desire to grow intellectually or with true leisure time, but at the intellectually curious, even philosophically educated, among the ruling elite or would-be elite. It reminds me a bit of the purported intended audience ["the philosophical reader" (philosophe lector)] of Spinoza's Theological Political Treatise.
The passage quoted at top of the post, occurs in the midst of Al-Farabi's fascinating (and to modern eyes unusual) treatment of modality. In fact, as an aside, just before justice was explicitly introduced, in the context of metaphysical multiplicity, as a metaphysical means of intervening in and settling (to simplify) the way a possibility is actualized or not (about that thought and the way Al-Farabi's cosmology and metaphysics is intrinsically normative some other time).
The quoted passage entails (animal rights' activists will not like this) that mankind has dominion over (most) other animals because they lack a rational faculty. But it also implies that by nature humans may not be treated by each other as a means because they have -- to use poetic language -- the spark of rationality. Let's call this Al-Farabi's rational humanism. It is compatible with his assumption that mankind is naturally hierarchically differentiated in many different ways and, in turn, cultivated and educated in ways that increase multiplicity.
Even so, within a political order, a truly wise ruler, the godly magnanimous soul, who is akin to a Platonic philosopher-king (and who combines theoretical and practical rationality), and (I believe) only the truly wise ruler can use and guide others as a means for their own good and happiness which will coincide with the city's good and happiness. (Al-Farabi is relatively elaborate on the nature of this truly wise ruler and rather terse on the institutional arrangements required for this rule.) That is, he can force others -- through education and cultivation -- toward the perfection of their own rationality to the best of their ability;--a people ruled by such a ruler (who emanates, as it were, his wisdom to his subjects) would be virtuous.
Al-Farabi does not say if such an exemplar has existed or will exist. But he does say we ought to think of such a ruler as a recipient of revelation, that is, a true prophet.