 Fârâbî gives no further explanation of how or why the existence of X can be a sufficient cause of the existence of Y. (He says that the existence of Y “emanates” [using the root f-y-ḍ] from the existence of X, but this is just a technical term meaning that X’s existence, rather than some further act of X, causes Y to exist; the term “emanation” does not explain how this happens, and Fârâbî does not pretend that it does.) He just infers that this must happen in some cases, since more than one thing exists. Fârâbî’s situation is analogous to Aristotle’s in Metaphysics XII. After Aristotle has argued that the existence of motion presupposes a first cause which is essentially pure actuality, and which therefore cannot be changed or moved, he faces the problem of explaining how such a cause can move something else. Aristotle answers that while an eternally unchanging cause cannot, by itself, cause changing motions, it can cause an eternally constant motion (such as the eternal rotation of a heavenly sphere around its axis) if it causes it as a final cause, as an object desired by the object in motion. This leaves many puzzles—why should this desire lead a heavenly sphere to move in circles, how will that help it attain the desired object—but Aristotle can say that since every other kind of causality is impossible, this is what must be happening even if we don’t understand the details. Fârâbî faces a similar difficulty in explanation, but he finds Aristotle’s solution unsatisfying, not so much because these details are missing, as because it does not explain why the heavenly spheres, or their incorporeal movers other than the First exist—at best, Aristotle assumes that the incorporeal substances and the spheres exist, and explains why the spheres move. Fârâbî, by contrast, has undertaken to understand the First as a cause of existence; and final causality, even if it is sufficient to explain the motion of an already existent substance, does not seem sufficient to explain the existence of substances other than the First, either the heavenly spheres or their incorporeal movers. The First must cause existence before it can cause motion, and in fact Fârâbî does not make it a mover at all, but rather says that it is responsible for the existence of something which is in turn the first cause of motion. And if the First causes existence at all, it must first cause the existence of at least one thing simply by its own existence, not cooperating with any further attribute or motion or instrument or matter. Fârâbî does not and cannot say anything about what the First does to cause the existence of something else (he does not say that it causes the existence of something else by thinking that thing, or even that it causes the existence of something else as a byproduct of thinking itself); rather, he tries to describe how, and in what order, other things are essentially dependent on the First.