It is difficult to imagine that there is an account of the interaction of the physical with the mental that has not yet been mapped out in the existing (vast!) philosophical literature on the mind-body problem, and that would be new to us. Yet, this is what I argue is the case with Plotinus’ account. He develops his innovative metaphysical position to explain the possibility of causal interaction between entities that he recognises to be categorically different, and causally inert towards each other – the soul and the body – and then applies his original account to solve the problem of how our soul can perceive external objects. His solution is to bridge the causal gap between categorically different beings by reducing the gap by half: he introduces a type of intermediate entity – the sense organs – which are qualitatively semi-mental and semi-physical.
The problem Plotinus starts from is that the soul, alone by itself, has no means of accessing and registering the presence and qualities of external objects. Importantly, Plotinus does not tell us that the soul cannot be affected by external objects; but rather that the causal route from external object to the soul is not a “proper” causal route, on account of the categorical difference existing between soul and objects. Therefore, his quest for an account of perception becomes a quest for a viable causal route from external object to soul, which becomes at the same time a quest to overcome their categorical difference.
This brings us to Plotinus’ metaphysical innovation concerning the mind-body problem. The challenge is formidable. The soul cannot be affected directly by physical objects, and so perceive them in this way. The blocking factor is that the soul and the physical objects are categorically different, which does not allow for a direct causal interaction between them. Plotinus’s innovation is to reduce the categorical gap by half, positing a type of entity which is, primitively, ontologically intermediate between categorically different entities – placing sense organs as intermediaries between the soul and physical objects. Through the senses, qua ontological intermediaries between the mental and the physical,the soul can establish causal connections with external objects.
The concept of ontological intermediary seems strange to us, as it entails ontological dependencies which do not make sense to us – we do not posit in-betweens in our ontologies. Nevertheless, the notion was first introduced by Plato, with regard to the ontological status of numbers, as Aristotle tells us in the Metaphysics. Objects of mathematics, e.g. numbers, are for Plato intermediate entities between the Forms and the physical objects, in so far as they share some of their features with the Forms (e.g. being eternal), and some of their features with the objects (e.g. there being many of a kind, by contrast to Forms, which are unique per kind). This seems to be the conception of intermediate which Plotinus employs.
Plotinus assumes that the causal intermediary between the soul and the objects of perception has to be similar to both, in order to allow interaction between them. To be similar to the categorically different beings of which it is the intermediary, the sense organs have to bear sensible and intelligible properties. This leaves open the question of how Plotinus thinks this could be achieved.
Plotinus expresses his view most briefly, in a key line. He writes that “This [third thing, i.e. the sense organs] must be able to assume the modifications [of the physical object] so as to resemble it, and it must be of one matter” (IV.4.23.22-23; my translation and emphasis). Plotinus tells us that the intermediary must be able to be causally affected by the sensible properties of objects in the world; to be qualitatively similar to both the objects and to the soul; and to be of one matter. I take it (by inference to the best explanation) that the latter requirement is what explains how the sense organs are intermediate between the two, objects and soul. Plotinus’ thought, I submit, is that the type of matter of which the sense organs are constituted enables them to bear properties that make them like the soul and like the sensible objects, because the matter that makes up the intermediary can take on both intelligible and sensible forms (forms of the soul, e.g. perceptual awareness of blue; and forms of physical objects e.g. the colour blue).
Which kind of matter can serve as the substratum of categorically different properties? I argue that Plotinus develops a sui generis metaphysical conception of the matter of a sense organ. The matter in question, I submit, is conceived by Plotinus as a special type of mixture of the sensible and the intelligible. What type of mixture can this be?
Plotinus does not make any explicit reference to Aristotle’s theory of mixture, but in absence of any account of his own, for the oneness of the matter in a sense organ, I submit that Aristotle’s theory is a helpful model to introduce us to how Plotinus conceives of his mixture of sensible and intelligible stuff. In the De Generation et Corruptione, Aristotle undertakes to explain the metaphysics of mixing elements; he innovates metaphysically, by showing that there is a way in which the mixed elements can be both present in, as well as absent from a mixture.
Importantly, on Aristotle’s account when elements of equal strength mix, they “survive” mixing; the items that are mixed are not destroyed in the mixture, but only compromised in their nature. In general terms, mixtures of this kind are constitutionally uniform, but are also complex compounds, because the mixed elements survive in the mixture in potentiality. The uniformity of mixtures is the crux of Aristotle’s theory of mixing, which makes his account an apt starting point for how Plotinus conceives the matter of the sense organs. The sense organs’ matter is uniform, just as the matter of Aristotle’s mixtures; but in Aristotle’s mixtures, the two mixants, which are of different types, are somehow present in it (in potentiality, and can be retrieved). For Plotinus, the two extremes are present, not as parts, but as aspects of a uniform stuff of the sense organ. These aspects are both of the same ontological level – there is no constitution or supervenience relation between the physical and the mental properties of the uniform stuff.
Thus, for Plotinus the sense organs are half-physical and half-mental. They are so on account of their matter, which is uniform, in the sense that every part of it is of the same type, but also, the type is simple in the sense of not being a compound of many, even if it is made out of many; and yet it is not the causal result of a process of composition. Plotinus is assuming that the mental and the physical can be found in nature fused or blended together, in the sense organs, even if they cannot interact causally between them. This fusion must be (ontologically) a primitive in nature, in Plotinus’ world, since there is no causal interaction between them to generate it, and yet they are not (like) an alloy, but a fusion or blend of elements.
It is the sense organs matter’s oneness and lack of internal complexity that makes it possible for Plotinus for the soul, via the sense organs, to respond to the external objects’s affections and perceive them. The intermediary could not be of one matter in the sense of being a heterogenous mixture constituted (literally, containing) two categorically different elements, analogous to, e.g., a barley and lentils mixture. This would be mixture by juxtaposition only, and although it would have qualities of both mixants, it would do so only by virtue of containing parts of both of the extremes. A heterogeneous mixture of intelligible-stuff and physical-stuff would replicate the categorical problem that such a mixture was posited, per hypothesis, to solve.
Aristotle does provide a metaphysical account of how uniformity is possible in his mixtures, which rests on the assumption that the properties of the two mixants can affect and compromise each other – e.g. the sweetness of honey and the sharpness of wine compromise each other into half-sweet/half-sharp. This capacity to compromise each other, crucially, is what is called into question when the properties of the mixants do not belong to the same category of being, e.g. temperature or weight, as they do not in the case of the physical and the mental properties. This difficulty is not raised or addressed by Plotinus. Yet, Aristotle’s theory of mixing aids our understanding of Plotinus’ views, and gives us a way to comprehend how he may have thought of the fusion of the mental and the physical, as if they were opposites, having degrees of difference in-between.
Through the fusion of soul-stuff and physical-stuff, a communication route opens between the soul and the external objects, which allows for mind-body mediated interactions. The question for us is: can we understand such cross-categorial blending? I am not interested here in the plausibility of the idea that such blend is possible in the case of the mental and the physical, but in whether Plotinus’ conception is comprehensible or not.
In favour of Plotinus, I will only mention that, if we can make sense of the mental having supervenience relations to the physical; or, on different theories, of the mental being physical; or of the mental not being physical; or of both being aspects of one and the same thing, why could we not make sense of other types of compresence relations between the mental and the physical, such as Plotinus conceived? Positing an intermediary type of entity, such as the sense organs, is a significant theoretical development on the part of Plotinus. He uses it to explain fundamental cognitive phenomena that otherwise remain puzzling. His solution rests on the metaphysics of intermediary entities, which is only briefly sketched by Plotinus, probably drawing on Plato. Plotinus’ assumption that the blend of different types of entities can be uniform can be understood in the light of Aristotle’s account of mixing.
Plotinus’ solution to the categorical gap problem, between the external physical property and the awareness of it in the perceiver’s soul, is to posit a causal intermediary that can resemble the physical sensible and can also inform the soul. This is made possible by a substratum, which is, I have argued, a uniform mixture of mental and physical stuff that can interact with both, the object and the soul, thereby grounding physical properties and mental content.
Has Plotinus thus solved the categorical gap problem that he set out to address in this theory of perception? There are some outstanding philosophical issues with Plotinus’ account of perception of objects that I want to raise. The first is that the matter that Plotinus posits as substratum of the sense organ comes with the cost of an additional primitive in his ontology. Since, as he says, the soul cannot by itself interact with physical objects in the world, the intermediate mixture of mental and physical in the sense-organ cannot be a product of soul-object interaction. It must be primitively existent in nature. Yet, how is this intermediate mental-physical mixture constituted? Are we to understand its composition in accordance with Aristotle’s principle, that opposite properties can compromise each other in mixtures, which, though, does not apply to categorially different properties – or does it? Further, Plotinus assumes that positing an intermediary whose matter is in-between mental and physical stuff somehow reduces the existing categorial gap between the soul and physical objects. It is as if these are conceived of as extremes on a spectrum, where the intermediary blend is midway between the extremes. As mentioned above, this presupposes that the intermediary differs from the categorially different extremes only by degrees. The question that remains open is how can two categorially different beings be extremes on a spectrum – a spectrum of what; degrees of what? Are all these theoretical costs that Plotinus’ theory incurs, or promises of a novel understanding of the mind-body problem, one of the hardest problems in philosophy?