The context of this short passage seems to be the beginning of chapter 10, in which Aristotle states, “we must also investigate in which way the nature of the whole possess the good and the best—whether as something separated and intrinsic, or as its order. Or is it rather both ways, like an army?” (1075b10-13). While Aristotle uses this as a starting point to discuss many complex theories about how the transcendent bears relationship to the immanent and how the universe is ordered around the good, the central questions seem to be thus: is the unifying principle of the cosmos one or many? And is this unifying principle separate from the cosmos, is it what orders the universe, or is it both?
After much discussion, he seems to settle on the notion that the unifying principle must be one rather than many (such as the forms or numbers), because, as he states, “for those who posit two starting points, there must be another starting-point with more control. And for those who posit the Forms, there will be yet another starting-point with more control. For why did things come to participate [. . .] in the Forms?” (1075b16-19). In this, Aristotle seems to regard “starting-point” not merely as the first cause of motion, but as the principle and “control” which unites all things, the thing that enables “participation” in the good and the best. For there to be unity, order, and the possibility of the many participating in the one, there must be a fundamental one.
From this, he progresses to his final argument of the chapter, insisting that what makes numbers one, and what joins soul and body, form and thing, must be the Prime Mover (1075b35-36). The problem, however, is that Aristotle does not explicitly show here how the Mover makes them one. He may be referencing back to a short argument in Book Eta chapter 6 in which he explores the cause of numbers being one and the cause of beings that seem to have parts but also be one thing (1045b6-7). In this, he focuses on his distinction between actuality and potentiality, matter and shape, insisting that the “essence” of a thing is the one that joins actuality and potentiality (1045b21-32). The analogy in numbers, presumably, would be the insistence that it is the “unit” or “oneness” that is the essence of everything called number by which numbers can be distinct and yet all called number, for they all share the one. Thus, analogically, just as each number is not what it is because is shares in the form of that specific number—but rather it is what it is because of its actuality and potential rooted in the nature of oneness—so too, a thing’s being doesn’t share in separate Forms, but rather finds its cause and nature in its actuality and potentiality rooted in the oneness of the Prime Mover. This in some ways remains mysterious, but it may be akin to what Aristotle refers in this sentence.
Aristotle focuses on the problem of number in the next sentence at the end of Book Lambda, insisting that “those who say that mathematical number is primary, and that in the same way there is again and again some other contiguous substance and a starting-point of each, make the substance of the universe episodic [. . .] and posit many starting points” (1075b36-1076a2). In this, he may be referring to the Pythagorians who posited everything to be number and Speusippus, who, as discussed in Book Zeta, believed there to be separate starting-points for each kind of substance, “one for numbers, another for magnitudes, and then another for the soul” (1028b21-23). This may refer back to his beginning discussion in Book Lambda, where he asserts that “if in fact that universe is a whole of some sort, substance is its primary part” (1069a19). In this case, the substance cannot be parts, numbers, or from multiple starting points, for the universe would then be subject to either infinite regress with no fundamental starting point, or it would be pulled apart by opposites, such as the problem with the Pythagorean claim that the one is made up of even and odd and the universe shares in contraries (986a15-986b1). This may also relate to his critique of Pythagoras and Speusippus in Lambda chapter 7, in which they state that “the noblest and best is not present in the starting-point,” for Aristotle claims that just as the seed comes from “the complete thing,” so the Prime Mover is the substance which is the one starting-point, “eternal and immovable and separable,” without parts or magnitude (1072b30-1073a5).
This leads to Aristotle’s conclusion at the end of Book Lambda: “Beings, however, do not wish to be badly governed: ‘To have many rulers is not good; let there be one ruler” (1076b3-4). In this statement, Aristotle seems to bring together many of the strands in Book Lambda, asserting that the Prime Mover is the fundamental substance which orders and unifies all things, but which is outside and separable from the universe, causing it to move and to be, not out of verbal declaration or its own movement, but out of the love of the universe toward it, a Mover yet unmoved, the one that causes the whole and yet separable from it. In this way, it is both “separate” and also “its order” (1075b10-13). It is what makes numbers one, what gives oneness to soul and body, form and thing. It is in every sense of the word prime.