After On Motion, this is the second subsection of Dodds' section titled "Of the Grades of Reality." Now we're really getting somewhere: having established the One and various kinds of motion, we can now differentiate various "levels" of reality by those kinds of motion. I suppose I should note that these "levels" aren't the same as the "planes" discussed by modern Western occultism. (For example, the level of "bodies," here, corresponds to the "material," "etheric," and "astral" planes in occultism.) It is important to understand how each level is distinguished from the next in order to map it to other systems!
On the Levels of Reality: From the previously-established propositions, we can deduce four distinct levels of reality: first is the One, which is unitary; second is intellect, which is unmoved; third is soul, which is self-moved; fourth are bodies, which are externally-moved.
XVIII. Every thing whose presence lends a quality to some thing must essentially possess that quality.
Suppose a fire causes a pan to get hot. It cannot be that the fire only communicates heat from some third thing to the pan, since the cause must be better than the effect (rather than the same) [VII]. It also cannot be that the heat of the fire and the heat of the pan are the same in name only, but differ in every other respect, because how can an effect be completely unrelated to its cause? No, the fire must cause the pan to get hot because fire is hot: the cause must essentially possess the quality that it lends to the other thing.
XIX. Every quality which is essentially possessed by things in virtue of their nature is present in every thing with that nature.
If only some things with a given nature possess a quality essentially, then the other things with that nature must possess it only accidentally; but if a thing only sometimes possesses a quality and sometimes not, then it cannot be a part of its nature.
XX. The soul's essence is prior to every body; intellect is prior to every soul; the One is prior to intellect.
We see some bodies move on their own (e.g. animals) while others don't (e.g. rocks); indeed, some move at one time (e.g. while alive) but not at another (e.g. while dead). It is clear, therefore, that self-movement is not an essential quality of bodies [XIX]. Self-movement must therefore be a quality lent to bodies by an external thing: we call this "soul," which must possess self-movement essentially [XVIII]. Since bodies are externally-moved and souls are self-moved, souls are higher than bodies [XIV].
Because souls possess self-movement, souls are capable of returning to themselves [XVII]. But since some souls are directed towards bodies while others are self-directed, self-direction is not an essential quality of souls [XIX]. Self-direction must therefore be a quality lent to souls by an external thing: we call this "intellect," which must possess self-direction essentially [XVIII]. But self-direction implies a lack a movement, since a thing directed upon itself already has itself and therefore has no need of moving to get it. But since intellect is unmoved while soul is self-moved, intellect is higher than soul [XIV].
Finally, despite a lack of motion, self-direction still implies a subject and an object: even if it loops back on itself, the "arrow" of direction must have a starting and ending place. This makes intellect have an essential duality; but every many participates in the One [I], so the One is higher than intellect. We can stop here: since the One is the first cause [XII, XIII], there is nothing higher than It.
I have altered the form of XVIII from that of a logical argument to that of an analogy, since frankly this was the only way I could follow it: Proclus says nonsense words like "thing" and "it" so many times that my head spins and I get lost. I took the analogy from Plotinus (Enneads I ii 1).
While most of the Elements has been a bit of a slog thus far, XX was genuinely fun and I enjoyed trying to work it out on my own from all the pieces given to us before reading the body of the proposition. I'm glad I did, since I think Proclus skips steps: in particular, I think incorporating XVII is necessary to glue everything together, but Proclus doesn't explicitly reference it. Dodds, too, says it's not used until CLXXXVI and speculated about how it must be placed so early in the text in order to refute the Stoics or something. Whatever—I think the logic of the proposition demands it, so either Dodds is off in outer space or else I'm missing something. Proclus himself speaks of intellect in terms of "thinker" and "perpetual thought", and says that this is how it differs from soul, but I'm pretty sure this is equivalent to self-direction (e.g. Plotinus calls it self-contemplation, and notes intellect's intuition is only possible when subject and object are unified), and anyway switching terminology like that makes the logic of the proposition harder to follow. Either a copyist made a mess of an early manuscript; or else Proclus is bored with this introductory stuff and rushed it; or else, again, I'm missing something.
Incidentally, Proclus likes to work from the top down: for instance, in his Commentary on the First Alcibiades of Plato, he says that "it is always requisite that demonstrations should begin from things more universal, and proceed from these as far as to individuals. For this mode of proceeding is natural, and is more adapted to science." So, I think it's interesting that, while he establishes the One first, he demonstrates the various levels of reality from the bottom up! (This is all the more striking since Plotinus started in the middle!) For whatever it's worth, though, I think bottom up makes the most sense in this context.
Despite a lot of playing around during my Plotinus read-along series, I keep the usual term "intellect" for the second-highest principle. (I don't like the term since intellect doesn't think: rather, it intuits or knows. Dodds notes that the German intelligentsia of his day had drifted towards translating it as "spirit," but, if anything, that seems worse. If one twisted my arm, I might suggest "consciousness," but this is cumbersome, and anyway if we're going to learn a weird term, we might as well use the weird term everyone else uses.) It is not capitalized (like it was for Plotinus) because—spoiler alert!—we have not yet established it's uniqueness. (I assume Proclus does this later on, anyway. We'll see!)