Plotinus was also a consummate logician in the sense of universal logic, ontological logic or logical ontology. Buried deep and intricately interwoven within the Enneads we find a sophisticated Science of Logic in the sense of Hegel but expressing a radically different spiritual and philosophical framework and outlook. We will not discuss here the interesting question of a direct influence on Hegel or of Hegel having directly and extensively appropriated neoplatonic doctrines in his own system (we have already given some references concerning this topic). Nor will we discuss the radical rupture in the philosophy and spiritual practices of the neoplatonic school that occurred after Iamblichus. We can only stress again the importance of expounding systematically a non-Proclean and indeed anti-Proclean neoplatonism.
The project we sketched in our paper "Hegel and Modern Topology" was to given a Hegelian interpretation of key aspects of modern mathematics, logic and computer science and to in turn use these to furnish an interpretation of Hegel (the hermeneutic virtuous circle). We find that most of the insights contained in what we sketched of this project can be transposed from Hegel's Logic to Plotinus's Logic where they seem to find a even more adequate and natural setting. But it must be understood that, as we explained before, in this framework mathematics and formal structures function essentially as an elaborate system of symbolism and hierarchical reflection rather than as candidates for a direct and complete formalization. The other aspect of mathematics and logic, culminating in dialects to be used as a tool for direct spiritual ascent, has to be considered separately.
In order to embark on this project we should list the fundamental categories, concepts and processes of Plotinus' Logic which we can organized somewhat along the lines of Hegel's Science of Logic but with fundamental differences which are often the complete inversion of Hegelian conceptions or which collapse what Hegel separated or separate what Hegel joined. Bearing in mind Hegel's Logic of Being we can remark that Plotinus has an elaborate and deeply interesting theory of quantity, extension, number, quality, essence, determination and measure (i.e. the interaction between quantity and quality found in VI.1-3, VI.6 and II. 6) which permeates in the Enneads (and justifies our classification of Plotinus as a finitist) - together with a remarkable theory of relation which when deployed at the intelligible realm fits exactly our considerations of groupoids as models for essential relationality and interconnectedness. The theory of a self-moving number and relationality of the nous fits well with our previous considerations on Galois groups and Galois extensions. Also the intelligible realm can be symbolized or illustrated as the category of relations.
We note that Proclus's theory of Henads is found in VI.6 but with the difference that for Plotinus they are a proto-determination in Being (as an infinite ouflow) which preceed the intelligibles rather than the One. For a form to exist as a distinct entity it must be bounded, structured, and distinct from other forms. Substantial number is the internal articulation that establishes this multiplicity within unity. In Plotinus’ view, number precedes being: intellect must possess the principle of number to differentiate being into distinct forms. Thus Proclus got his theory of henads from Plotinus' theory of essential number.
And also a highly developed and articulated theory of infinity (outflow, power) and limit (reversion, theoria) and a theory of "matter" (both physical and intelligible) which amounts in the physical level to the poorest and emptiest determinations of being (strongly echoing the initial stages in the Logic of Being). Indeed in II.4.9-10 (cf. also VI.6.2-3) Plotinus invites us to perform an abstraction of the quantity and all qualities of a body to arrive a the concept of indetermination. Matter has existence and a disposition to become other things. Intelligible matter is equated to being and intelligible apeiron (the infinity of the One) and physical matter to non-being and to physical apeiron.
Plotinus states explicitly that indeterminate matter will display contradictory dualities - a striking possible interpretation of quantum theory which becomes less paradoxical once we abandon a foundationalist reductionist framework for a top-down emanationist one.
Plotinus' theory of quantity and quality agrees admirably with the framework of modern topology (he recognizes geometry as a kind of quality in quantity) and differential geometry specially as applied in field theory and differential equations. Indeterminate matter first must receive extension and it is only upon extension that quality can be received. This corresponds exactly to the primordiality of a base space (space-time manifold) relative to its fiber-bundle or sheaf which expresses the local physical-phenomenological properties of the field (Plotinus considers intensive quantities like temperature to be qualities). This lower kind of qualitative structure in turn is only the product of a higher structure and power. For Plotinus space (extension) itself is a product and emanation of higher structure just as a non-commutative $C^\star$-algebra may have within itself various commutative (maximal) subalgebras.
Thus sensible non-essential quality corresponds closely to Hegel's concept of measure, as quantity determined by something from without and as qualified quantity. The first section on Quality in the Science of Logic corresponds rather to the genesis of quantity in the intelligible realm and soul, where we deal with united multitudes, intelligible forms and their relations and overflow and limit (and we can include indefinite quantity here). Hegel's subsequent treatment of quantity and measure corresponds to sensible quantity and quality in Plotinus and is quite adequate for the mathematical illustrations in Hegel and Modern Topology.
Intelligible quantity exists as pure number and structural order (the archetype of limit and harmony. Intelligible quality exists as the specific power and form (eidos) of an Ideal entity. At the higher intelligible level, quality and quantity do not conflict or merely sit side-by-side; they are unified aspects of the divine Forms. When projected down into the physical world, they split into extended magnitudes (sensible quantity) and superficial properties (sensible quality). In II.4 we have that pure matter has no magnitude or spatial extension of its own. Extension (megethos) is a form or quality superimposed onto matter. In VI.4-5 dealing with omnipresence we read that the Soul is not spatially spread out or extended. Instead,
physical extension is a limitation, whereas incorporeal reality
possesses a unified, non-particulate presence that contains spatial
extension without becoming spatial itself. In III.7 we read further that just as time is an unrolling or diastasis of the eternal life of intellect into succession, physical extension (megethos) is a dispersion or unfolding of unity into divisible parts.
A fundamental mathematical symbolism or reflection will be that of the limit (let us say, in the category theoretic sense) of a sequence or diagrams expressing an indeterminate outflow or process. This limit is a reversion, completion, perfection and theoria in the Plotinean sense and an aufhebung and being-in-and-for-itself in the Hegelian sense. We can think of this process as dynamic and limit and reversion as a fixed point (cf. Lawvere's account of Gödel's incompleteness theorems and diagonalization arguments).
The reversion of the physical word and the human soul to the level of Soul and Nous parallels to a certain extent to Hegelian levels of Essence of Concept (for instance the supreme genera in the intelligible realm include identity, difference and in VI.3 rejects that essence or ousia could apply to the physical world) but with many radical differences and fundamental corrections from a neoplatonic point of view. The physical plane can be considered in itself or as it is the result of the activity, influence and emanation of the the plane of soul taking account its act of reversion (i.e. Objective Concept, the theory of life) - and the same goes for the the plane of soul itself. Thus Subjective Concept can correspond to the theory of the soul's logoi, to the theory of the world soul or to such as it relates to the intelligible realm and to the intelligible realm in the aspect of knowledge. Hegel's progression from outwards qualities to essential qualities in Subjective Concept finds its parallel in VI.1.15–22 which distinguishing true intelligible qualities (forms or reason-principles - logoi) from mere sensible qualities (passions, colors, surface shapes). For Plotinus universal Being (i.e. the Absolute as the perfect emanation of the One) is Being, Intelligence and Life and the perfection of energy/act/power.
When using mathematical structures as neoplatonic symbols and reflections we find that it is best for formulate them and think of them in a strictly constructivist way. Thus the number of objects in a category must be conceived as being indefinite and defined as one would do in a dependent-type based proof assistant - considering it to be the object object in the definition of an internal category. The finitist can consider intentionally the sequence $\frac{1}{n}$ as computable process without having to admit the finished extended set of such numbers. And then the finitist can consider $0$ as being its limit.
The considerations presented above are very sketchy and highly insufficient. We point out the the deep exposition of Plotinus' logic can be found in VI.2 and VI.6. The five supreme genera suggest the fundamental importance of the equality type and transformations which preserve structures and express coherence and relational unity or interconnectivity.
Identity is related to stability, difference to movement. Consider the two possible orientations of a vector space. They are indistinguishable, intrinsically identical yet different. Their difference is expressed by a non-trivial automorphism of the underlying vector space which transforms one into the other.
Plotinus states that number is produces by a combination of movement and rest (as supreme genera) and that extension is posterior to number and is the result of a movement produced by infinity which is halted and limited by rest. In VI.6 we have the theory of multitudes as outflowings and divisions of being. In VI.6.1 we read that if the parts maintain some connection then we have extension which have lost perfection must receive the form of beauty. The negation of negation is the reversion of the outflow through limit. Negation is related to set-theoretic complementation (for instance it is the interior of the complement for the Heyting algebra of regular open sets of a topology). Thus the dyad, the unlimited, indeterminate outflow, considered in itself independently of its source can be characterized as negation. The negation of negation (a reversion of the outflow) corresponds to a closure operator which encompasses in a limited determined way the object which was initially negated. If negation corresponds to the sun's unlimited rays then double negation corresponds to a halo around the sun's disk.
The remarkable sections VI.2.4-12, which deduce the supreme categories through an anagogic process of turning inwards, the key concepts being soul, the logoi. one, ousia, life, intellection, reversion, movement and stability, identity and difference, are important for comparison with the Logic of Essence and Logic of Concept and well as for pointing out the radical difference with Hegel. At the same time Plotinus' henological ontology and the deduction of the other genera from the supreme pentad in VI.2.13-21 should be compared to the processes and structures of Hegel's Logic of Being. We note the remarkable fact that VI.2 (which employs many argumentative structures interesting from the point of view of formal logic) can be seen as realizing the idea of dialectic which Plotinus sketched in I.3. In VI.6.10 we read that Being became Number when it became a multitude, having had a pre-formation, previous representation,or allocation for the beings it produced. We note that Augustine copied VI.6.12-13 almost directly. A section in VI.6.3 should be compared to Hegel's absolute negativity in the beginning of the section on Essence: τὸ δὲ μὴ ἀποδρᾶναι ἔχειν, εἴργεσθαι δὲ ἔξωθεν καὶ κύκλῳ καὶ μὴ ἐξεῖναι προχωρεῖν, στάσις ἂν εἴη: ὥστε μὴ μόνον ἐξεῖναι κινεῖσθαι λέγειν.
We note that a fundamental mathematical illustration of the genesis of sensible quality will be that of the unfolding of a singularity (or deformation of a structure). We must bear in the mind that intelligible extension is expressed as the intensive power of a intelligible form which then unfolds and expands spatio-temporally in the world-soul. Or the forms can be symbolized by infinitesimals as we developed in "Hegel in Modern Topology" - and indeed in V.9.6 Plotinus gives us an interesting illustration involving the logoi and a seed.
Note the Hegelian categories of ground and appearance, phenomena, etc. find their treatment in Plotinus theory of sensible substance which is a shadow produced by the Soul casting form onto receptive, featureless matter. Physical substance is a composite (synolon) of matter (hyle) and dynamic immanent form (logos or reason-principle). Physical motion is the life and process of change inherent in time and physical nature. It is the physical shadow of the eternal activity (energia) of Intellect.
Consider an object $A$ in a category. Then $A$ may have internal symmetries in the sense that $hom(A,A)$ may contain more than just $id_A$. Given $f \in hom(A,A)$ we can ask if there exists $g : A \rightarrow B$ and $h : B \rightarrow A$ where $B$ is not isomorphic to $A$ such that $f = h \circ g$. This offers an illustration of $B$ being an emanation of $A$ (flowing out in $g$ and reverting by $h$) and proceeding from $A$'s internal structure.