 A reminder, first, of what this primacy is. As we saw in Sections 2–3 above, only beings in the category of substance are separable, so that they alone enjoy a sort of ontological priority that is both existential and explanatory. Thus walking and being healthy are characterized as “incapable of being separated,” on the grounds that there is some particular substantial underlying subject of which they are predicated (Z.1, 1028a20–31). Often, indeed, separability is associated with being such a subject: “The underlying subject is prior, which is why the substance is prior” (Δ.11, 1019a5–6); “If we do not posit substances to be separated, and in the way in which particular things are said to be separated, we will do away with the sort of substance we wish to maintain” (M.10, 1086b16–19). Similarly, not being separable is associated with being predicated of such a subject: “All other things are either said of primary substances as subjects or in them as subjects. Therefore, if there were no primary substances, there could not be anything else” (Cat. 2b3–6). The starting-points and causes of all beings, then, must be substances. But for all that has been shown so far, the universe could still be made up of lots of separate substances having little ontologically to do with each other.