2. If the first principle of all were composed of parts, then those parts would be ontologically prior to it.
3. But in that case it would not be the first principle of all.
4. So the first principle is not composed of parts, but is absolutely simple.
5. If there were a distinction between what the first principle is and the fact that it is, then there could be more than one first principle.
6. But in order for there to be more than one, there would have to be some attribute that distinguished them.
7. But since a first principle is absolutely simple, there can be no such attribute.
8. So there cannot be more than one first principle.
9. So there is no distinction in the first principle between what it is and the fact that it is.
10. So the first principle is not only absolutely simple but utterly unique: the One
Let’s walk through the argument step by step. (The comments that follow in some cases go beyond what Gerson himself says.) What is meant by a “first principle” in step (1) is, essentially, a bottom level explanation of the world, something that explains everything else without needing an explanation itself. Accordingly, this premise is at least implicitly accepted by the atheist no less than by the theist, at least insofar as the atheist regards scientific explanations as terminating in a most fundamental level of physical laws that determine all the rest – whether this takes the form of a “Theory of everything” or instead a conjunction of several physical theories left unreduced to some such single theory. The dispute between Plotinus and the atheist, then, would not be over the existence of a “first principle,” but rather over its character. And Plotinus wants to show in the rest of the argument that the first principle of all would have to be simple in (something like) the sense of “simplicity” enshrined in the doctrine of divine simplicity.
The “parts” referred to in step (2), accordingly, are parts of any sort, whether material or metaphysical. The idea here is that if a thing is composed of parts, then the parts are more fundamental than it is. Moreover, those parts would need to be combined in order for the thing to exist. (This is true even if the thing has always existed – for there would in that case still have to be something that accounts for why the parts have always been conjoined.) A purported “first principle” with parts just wouldn’t be a bottom level explanation or first principle at all, then – it would in that case need explanation itself.
With step (4), then, we arrive already at the simplicity of the first principle of all. But when Plotinus refers to this principle as “the One,” he does not mean merely that it has no parts but also that it is utterly unique – that the sort of theism his argument leads us to is necessarily a monotheism. That is part of what the next stage of the argument seeks to establish.
It also seeks to establish an aspect of the doctrine of divine simplicity that is usually thought to be more distinctive of later, Scholastic philosophy. The distinction in step (5) between what a thing is and that it is is, as Gerson says, an anticipation of the famous medieval distinction between essence and existence. In things whose essence and existence are distinct – which, for Aquinas, is everything other than God – the essence entails a general category under which distinct instances might fall. There is, for example, the essence human being, under which Socrates and Obama both fall as particular instances, each with its own “act of existing.” (See chapter 2 of Aquinas for the rundown.) Similarly, if the essence of the first principle of all were distinct from its existence, there might be this “first principle of all” with its act of existing, that “first principle of all” with its own act of existing, and so forth.
But for that to be possible, there would, step (6) tells us, have to be some attribute that one “first principle of all” had that the other lacked. And that, Plotinus holds, makes no sense. For then it would be what they did not differ with respect to – what they had in common – that would be the true first principle of all, since it would be that which ultimately makes each of them the kind of thing it is. That is to say, one “first principle of all” and a second “first principle of all” would each be what it is only because each instantiates the same essence; and in that case it would be the common essence itself, and neither of the individual instances, which (as the explanation of these instances) would be the true first principle. Moreover, we would have in this case a distinction between a first principle itself and its attributes, which conflicts with the simplicity arrived at in (4). Hence there can be no such attribute (step (7)), and thus no way in principle to distinguish one first principle of all from another (step (8)), and thus no difference between the essence of a first principle and its existence (step (9)). The first principle of all is thus “simple” or without any parts in the strongest possible sense.
Actually, an essence/existence distinction would seem directly to violate (4), making the reasoning from (6)-(9) redundant; but as Gerson interprets him, Plotinus seems to argue in this manner anyway. In any event, if essence and existence are identical in the first principle of all – if the first principle isn’t a being among others in a general category but rather just is subsistent being or existence itself (ipsum esse subsistens, as Aquinas would later put it) – then we have something approaching the doctrine of divine simplicity as it would come to be understood in the classical theistic tradition.
We don’t yet quite have classical theism per se, however; for the One is, of course, but one of three divine “hypostases” in Plotinus’ view, even if the most fundamental. The remaining two are Intellect and Soul, and we will examine what Plotinus has to say about them in part II.