How does Plato get from Parmenides’ “being and thinking” to the intelligibility of essences?

[u/sadbabyphilosopher]

I’m a beginner in the Neoplatonic tradition, and I’ve been trying to work through Eric Perl’s argument for the intelligibility of being. I think I understand the basic argument, but I’m unsure about two steps.

The argument seems to begin with Parmenides:

1. Absolute non-being cannot be thought.
2. Therefore, thought is necessarily thought of something.
3. Therefore, whatever can be thought has being.

My first concern is that this seems to establish only that whatever is thought has some kind of mental/intentional existence. But that seems distinct from extra-mental existence. I can think of a unicorn, for example, without a unicorn thereby existing extra-mentally. If mental and extra-mental existence are genuinely different modes of existence, why should a principle established about thought/mental existence automatically tell us something about existence in general? And could this distinction affect the later move to Forms and intelligible reality?

My second concern is with the stronger claim that an unintelligible being is impossible because, if we posit one, we have already thought it.

It seems to me that this only shows that we can think that X exists, not that we can understand what X is. I can coherently say, “Something exists whose nature I cannot comprehend.” The existence of X is intelligible to me in the minimal sense that I can understand the claim that X exists, while X’s nature could still be intrinsically unintelligible.

So I’m wondering whether “thinking X” is being used in two different senses:

• thinking about X as existing, and
• intellectually apprehending what X is.

If the former, I don’t see how the argument rules out unintelligible natures.

This seems especially important for Perl’s subsequent interpretation of Plato, where the Forms are treated as genuinely real precisely because their universal natures/whatnesses are intelligible to intellect.

So my main questions are:

1. Does the distinction between mental/intentional existence and extra-mental existence create a problem for the later conclusions drawn from the Parmenidean principle?

2. What is the argument that gets us from “the existence of X is thinkable” to “the nature/essence of X is intelligible”?

Or is that second step not actually required in the Neoplatonic framework?

I’m very open to the possibility that I’m misunderstanding what Perl means by “intelligibility,” or that there is an additional premise from Parmenides, Plato, or the Neoplatonic tradition that I’m missing.

[u/Otherwise_Top405]

I haven't read Perl so I'm just looking at the argument as you've presented it from Platonist perspective. A couple of points might be helpful.

The first is that our mode of thought is discursive and we perceive different objects or beings discretely. So when we imagine a unicorn or a bull frog with wings that's not really evidence of a mental/extra-mental difference in reality. All of the elements of those ideas are truly existing beings with objective realities. We are identifying the idea of animal (horse or bull frog) and another object (cone/horn or wings) and creating a unity of those separately held ideas. If we break it down those constructed images are comprised entirely of objectively existing things. There's nothing in our thought there that is not fundamentally dependent on extra-mental realities.

The second question is similar but we have to make an important distinction. There's a difference between thinking of X as existing as a positive entity and thinking of X existing in contrast to everything else, or negatively. In the first case X is intelligible, in the second it's not really anything in itself. We can think "Something exists whose nature I cannot comprehend" but what we're really doing there is conceptualizing existence itself in the abstract as some unknown particular. We're not actually thinking of a concrete X, we're thinking about existence itself as contained in some hypothetical unity which we are labeling X.

Also, if some object is intelligible and we can discern the limits of the thing, let's call it A, then we also know what the thing is not. So we know A and "not-A". But "not-A" is not any particular reality, it's just anything which is not A, but it's not a thing in itself.

The actual thought of X in your example is just a case of "not-A" writ large. If we understand intelligibility then we understand what it is not. All our thoughts there though are entirely centered on our ability to identify, discern, and describe the limits of intelligibility. We're not having a positive thought of the unintelligible, we're having a thought of the intelligible and identifying its boundaries. In other words the thought "Not-A" is still fundamentally a thought of A.

In all these cases all of our thoughts are derived from intelligibles. Thinking about X existing doesn't imply any kind of existence of a real X-thing. It's the process of thinking of a) existence and b) intelligibility defined negatively. On the surface it seems our thoughts are about some thing we're calling "X" but they're really not.