One of the more difficult or troublesome aspects of the Aristotelian tradition for me is the notion of “separate substances”. Certainly this was greatly expanded by later writers, but it appears to have a basis in the texts we attribute to Aristotle.
A separate substance would seem to be something that at least does not participate in earthly matter, but in Aristotle there is some ambiguity whether separate substances are supposed to be completely free of matter, or just free of earthly matter. In Aristotle’s thought this makes a difference, because he regarded celestial matter as being fundamentally of a different kind than the earthly matter that is subject to generation and corruption. Conditioned as we are by theories of modern science, this may seem quaint to us, but it is a reasonable inference if we focus on ordinary human experience. In a similar way, Aristotle refers to the movements of the stars as “eternal”, on the practical ground that humans looking up from Earth do not observe them to change.
There is a more general point regarding eternity. Aristotle seems most often to use this term in the pragmatic way mentioned above. He calls things eternal if they are constant within human experience. This is quite different from the stricter sense of eternity, as meaning outside of time altogether. Plato’s geometrical objects and Augustine’s God, I think, are supposed to be outside of time altogether. Both the Aristotelian and the strict notion of eternity are different from the more popular religious sense of eternity as lasting forever. Medieval theologians coined a special word for the latter: “sempiternity”.
The separate intellect or intellects that Averroes talks about do not seem to me to be eternal in the strict sense of outside of time. At one point, Averroes says for example that while he assumes humans will always exist, if per contra humans became extinct, there would no longer be a material intellect. Aristotle had said that the potential intellect is nothing at all until it begins to think, and Averroes seems to say that humans provide the occasions for it to think. Having a dependency on temporal beings seems to me to be incompatible with eternity in the strict sense.
An even subtler question is how to regard the situation in which something strictly eternal is said to act as a final cause that is said to move something earthly. As noted in the last post, Aristotle regarded motion as something existing only in the thing moved. He distinguished “moved movers” from “unmoved movers”, but moved movers are moved by something other than themselves. For Aristotle, everything that is moved is moved by something else; there are no absolute by-their-bootstraps “self-movers”, although most or possibly all beings have some degree of activity of their own. So a moved mover is called that only insofar as it is moved by something else.
The preeminent movers in Aristotle are ends — not mechanical impulses, not generators of something from nothing, not “agents” of any sort at all. Ends have a kind of virtual existence or subsistence that could in principle be independent of any embodiment. They move us in the way that values move us. Aristotle suggests that even earthly matter is moved by ends in some rudimentary way — not that it has consciousness of its own, but that its being exhibits something analogous to preferences to being in one state rather than another. This is just what it is to “have a nature”. All Aristotelian natures have some involvement with ends in this way.