First, as a Platonist, Plotinus is committed to the existence of the Forms, for the usual sorts of reasons. (See chapter 2 of The Last Superstition for a refresher course on Platonism. Though the book ultimately opts for Scholastic realism rather than Platonic realism, I try to make it clear why Platonism is intellectually attractive.) Given the Forms’ existence, Plotinus appears to take the following line of reasoning to demonstrate the existence of his second divine hypostasis:
1. The Forms constitute a complex system of eternal necessary truths.
2. This system can exist only virtually in the One, not actually, since the One is not complex but absolutely simple.
3. So there must be a separate, derivative hypostasis to ground the actuality of these truths.
4. But eternal truths cannot exist except as contemplated by an eternal knower.
5. So this derivative hypostasis must be an Intellect.
Once again, let’s comment on the argument step by step (and once again, not everything I have to say by way of commentary should be attributed either to Plotinus or Gerson). Things are what they are because they instantiate certain Forms or universals. This individual triangle and that one are what they are because they instantiate triangularity, this individual cat and that one are what they are because they instantiate catness, and so forth. The Forms are, accordingly, essential to any complete explanation of the world. They are eternal and necessary – this or that triangle comes and goes, but triangularity does not and cannot. They are also logically interrelated in a way that entails various eternal and necessary truths – anything instantiating triangularity necessarily also instantiates trilaterality, anything instantiating threeness necessarily also instantiates oddness, and so forth.
At the same time, the Forms must exist “actually” if they are to explain anything. Hence, step (3) concludes, there must be a second-most fundamental explanatory principle of all reality after the One, in which the Forms are actualized – divine (since it is eternal, necessary, and part of the ultimate explanation of the world) but derived from the One. What is the nature of this “hypostasis” (to use the traditional Neo-Platonic language)? Anticipating a line of thought that would later become associated with St. Augustine, Plotinus argues that though eternal truths exist independent of the material world and independent of any finite mind (this much being familiar from Plato), they must nevertheless exist only as contemplated by some mind, namely an infinite mind (step (4), a move that arguably goes beyond Plato and makes Plotinus’ position a “Neo-” Platonism, though Plotinus himself didn’t see it that way). And thus we reach the conclusion that the second hypostasis must be thought of as a kind of divine Intellect.
But we are, in Plotinus’ view, still short of a complete account of the ultimate causes of the world. This brings us to his argument for Soul, the third of the three hypostases, which can be summarized as follows:
1. Intellect, since it contains the Forms, explains why things have the natures they have.
2. But the objects of the Intellect are not distinct from the Intellect itself.
3. And some things desire objects outside themselves (e.g. a plant “desires” to grow and flourish).
4. So Intellect cannot explain these desires.
5. So a third hypostasis is needed to explain them, namely Soul.
This argument might seem even more difficult to understand than the other two we’ve considered, but I think if we unpack it carefully we can see that it is not all that mysterious, at least given certain key metaphysical assumptions held in common by many ancient philosophers. Keep in mind that the point of Plotinus’ theory of the three hypostases is to show what must be the case if the world we know is to exist at all. It is to provide an ultimate explanation. The One is the ultimate explanation of the being or existence of things. Intellect is the explanation of their natures. But the world of our experience has, we might say, both its static and its dynamic aspects. The nature of a cat is what it is, unchangingly. As step (1) reminds us, Intellect is the explanation of why it has that nature. But as a living thing, a cat also goes through stages of development. It doesn’t exhibit every aspect of its “catness” all at once. In general, living things exist in a way that involves the gradual realization of various ends. Plants “desire” or “seek” to grow and flourish, acorns “aim” to become oaks, and so forth. Most living things do not consciously seek to realize such ends, of course. What we have here is best understood along the lines of Aristotle’s conception of final causality, “goal-directedness” in nature that is for the most part entirely unconscious and unthinking.
This “dynamic” side of the natural order is something Intellect does not explain. The reason is that goal-directedness or final causality involves one thing being “directed toward” something outside itself, and there is nothing like that in Intellect. As I have noted in several earlier posts (e.g. here and here), for ancient and medieval philosophers, thought does not involve the intellect’s “representing” the world outside it, but rather a kind of identity of the intellect with the nature of the thing it knows. This is why step (2) tells us that (the divine) Intellect is not distinct from the Forms it knows; in knowing a Form it does not “point to” something beyond itself, but is rather, in a sense, identical with the Form. (Again, see the earlier posts just linked to for more on this conception of thought.) Since there is no “pointing beyond itself” in Intellect, step (4) concludes that Intellect cannot account for those aspects of the natural world (noted in step (3)) that do involve a thing’s “pointing beyond itself” – that involve “desire,” goal-directedness, final causality. (Plotinus focuses on living things, but to the extent that final causality, properly understood, pervades the natural order in the ways discussed in many previous posts, all of material reality would seem to require such explanation for the same reason living things do.) There must, then, be a third-most fundamental explanatory principle of all reality after the One and Intellect, one which accounts for this “directedness” of things towards certain ends outside them, and this is what Plotinus means by Soul.