1.

Matter, the most generic term for what physicists study, has become puzzlingly elusive in scientific parlance today. Everything in the cosmos consists of matter, but what is that? Some matter, we hear, is ‘ordinary’, consisting of atoms, but most of it is ‘dark’ and is indescribable in the standard model of particle physics. Together with the so-called ‘dark energy’ permeating space, dark matter accounts for 95% of the world's mass-energy content. We can infer its presence from the behavior of visible matter, but dark matter as such (hence its name) is of unknown composition.Footnote ¹

Plotinus, the latest ancient Greek philosopher of paramount genius, would have sympathized with the modern physicists' difficulties in describing and defining matter. He even anticipated them in calling hyle, his own term for matter, ‘dark’ (skoteinos).Footnote ² Plotinus acknowledged the properties of perceptible matter, taking their presence to be straightforwardly evident from our experience of ordinary objects like trees, stones, and animals – bodily items, spatially extended, and endowed with qualities that impinge on our senses. If all that we mean by matter is the visible and tangible stuff of which ordinary objects consist – the wood of the bed, the bronze of the spear, the flesh and bones of the animal – there is no linguistic or conceptual problem for Plotinus. But what are we to say or think, according to him, when we extend the scope of matter beyond ordinary objects?

Is there a further matter of which perceptible matter, in all its variety, is ultimately composed, a more or a most primitive feature of the world's physical make-up? The favourite ancient answer to this question had been affirmative – to wit, a set of four elements, comprising earth, air, fire, and water. Earth, air, fire and water were generally understood to be qualitative terms, each of them referring not to a single, homogeneous stuff – water in the sense of H₂0 for instance – but to the following set of primary qualities or combinations thereof: hot, cold, moist, and dry, all of which admit of change and degree. Qualities of what, we may ask, to continue the analysis? The classic answer – the answer of the Aristotelian school-had been ‘prime matter’.Footnote ³

Plotinus's problem, and its nominal affinity to the dark matter of modern physics, can now be stated. In order to serve as the physical foundation of everything, absolute matter cannot be identified with definite or determinate things, such as the four elements. For in that case it could not serve as their foundation too. Matter as such or matter simpliciter, to use Aristotelian language, must always be potential and analogical rather than actual, drawing its descriptive identity from the composite things of which it is the matter – for instance the bronze of the spear, the wood of the bed, etc. Is there, then, any such entity as prime matter if that expression signifies a completely indefinite and indeterminate foundation for things? How can we know that such matter is in any sense real and not a mere figment or convenience of our conceptual scheme?Footnote ⁴ I want to show how and why Plotinus grapples with these deep issues.

He is not willing to say outright that matter is nothing. Matter is, in his view, a precondition for the existence of bodies, three-dimensionally extended and perceptible objects; but taken by itself, matter, according to Plotinus, is not bodily or physical stuff or anything with determinate quality or quantity. Taken by itself, matter is completely imperceptible and amorphous. We know it only by a kind of inference from phenomena. Try, as we may, to imagine matter independently of form, we find ourselves completely in the dark, trying to picture the least accessible of the world's contents. And yet, however hard it is to specify, matter is there and is necessary. So much by way of introduction to the issues this paper will raise.

2.

I have chosen to discuss Plotinus because, at this late stage of my career, I find him one of the most challenging (and certainly the most difficult) of all the Greek philosophers (to whom I was first introduced in the years 1958–1960, as a Classics student at University College London, a few steps away from the home of the Royal Institute of Philosophy in Gordon Square). Plotinus, working on his grand metaphysical scheme in the first decades of the third century of our era, resists dividing the world dualistically into thought and extension, or animate and inanimate, or body and spirit. These binary categories are inapt, he postulates, because every existent thing has its primary source in everlasting and transcendent Unity, which he also calls God or Father or The Good. For everything that exists there is a corresponding idea, derived from Unity; or rather, existence (meaning determinate, stable reality), depends on what can be thought. Here Plotinus echoes Parmenides, one of his most hallowed predecessors, whom he takes (correctly, in my opinion) to identify being and thinking. Can ultimate matter be thought? If it cannot, it must exist only in some equivocal sense. Ultimate matter, as envisioned by Plotinus, is equivocal indeed, but it is not an incoherent notion. Its obscurity is essential to the queer role it plays in his metaphysical scheme.

I will now give a brief genealogy of the notions of matter that Plotinus inherited from earlier Greek philosophers.Footnote ⁵ I will then work through a selection of his statements, taking them mainly from his short essay On Matter.Footnote ⁶ This procedure will enable me to ask in more detail, ‘What is the matter with matter?’ from his stance. In conclusion, I will ask whether the dark matter of modern physics and the darkness Plotinus attributes to ultimate matter have anything more than a name in common.

3.

Plotinus begins his essay with the expression ‘so-called matter’. This evasive-sounding phrase is appropriate because his Greek word hyle, conventionally translated by ‘matter’, starts its linguistic life meaning wood or timber; hence ‘material’ is often a better translation. Our English word ‘matter’ is derived from Latin materia. In Latin usage materia, like Greek hyle, primarily denotes wood or timber, deriving that name from the word mater in the sense of mother earth or parent.

From the outset, then, hyle is a metaphorical term standing generically for what things are made of (for instance wood) and then extended to signify the ultimate foundation of physical things. The earliest Greek cosmologists did not draw on the term, but they operated with comparable metaphors. Empedocles called earth, air, fire and water, which are his four primary beings, ‘roots’. Soon this quartet was given the name ‘elements’ (stoicheia), a term which is also a metaphor for the world's basic physical components, taken from the Greek word for the letters of the alphabet. ‘Seeds’ (semata) or ‘beginnings’ (archai) were other metaphors that early cosmologists drew upon, to try to capture a notion like our idea of basic matter in ordinary speech. The point to emphasize here is that matter, signifying the world's physical foundations, was and always has been a metaphor-a theoretical notion lacking any fixed empirical reference in itself. Hence we are able to keep using the term even when our scientific models have completely changed. You may care to reflect on the checkered history of modern attempts to give a final definite description to matter – the periodic table of elements; protons, neutrons, and electrons; quarks; strings etc.

Plotinus was familiar with the four Empedoclean elements and also with the atoms of Democritus and Epicurus. From our modern perspective these theories, especially Epicurean atomism, are among the most important antecedents of early European science with its understanding of matter as the ultimate physical make-up of everything. That is because elements and atoms were theoretical notions that envisioned the world's foundations in discrete corpuscular, or what we call ‘material’, terms. Plotinus, however, totally rejected corpuscular theories of ultimate matter. His cosmology, like that of Plato, Aristotle, and Stoic philosophers, invokes reason, structure, and design as the primary factors explaining the way the world is perceived to function. Unlike modern bottom-up, material models of explanation, with the world conceived as evolving from simpler to more complex states, Plotinus's philosophical ancestors largely proceeded by means of a top-down explanatory model. On their view, matter does not evolve, under its own causal power, into derivative elements and life-types. Rather, matter is taken to be the recipient of pre-existing forms or formative principles, and it is they (commonly called logoi, also translatable as formulae) that energize and characterize matter. (Thus, in Aristotle's understanding of sexual reproduction, the male parent's sperm endows uterine matter with specific animal form.)

Plotinus sets the scene by stating what he takes to be the shared view of Platonists, Aristotelians, and Stoics:

To unpack this difficult sentence, we need to clarify the words substrate, receptacle, and forms. All three of these terms are basic to Plotinus's own understanding of matter.

The first thing to emphasize concerning Text 1 is the absence of physical determinacy from this preliminary account of matter. Plotinus, as he continues, states that some of those who share the substrate and receptacle theory (he means Stoic philosophers) do attribute ‘body’ to matter, whereas other philosophers (Platonists and Aristotelians) take matter as such to be ‘bodiless’ (asomatos). However, the Stoics' corporeal matter, is not an empirical entity but theoretical plasticine, as it were. This is how it is described by Calcidius in his commentary on Plato's Timaeus:

Here we can see how Stoic matter, corporeal though it is, fits Plotinus's generic description of matter as a substrate and receptacle of forms, i.e. a foundation of reality that is completely inert and amorphous.

‘Substrate’ (in Greek hypokeimenon) is an Aristotelian word. Plotinus explains the term as follows:

The ‘remaining’ item, what receives the form and persists through the change, is matter in the sense of substrate. Plotinus exemplifies this notion by reference to metallurgy – making a cup out of gold, and then smelting it down again: the cup comes to be and ceases to be, while the gold of which it is made persists. The gold, as substrate for the cup, exemplifies the proximate matter that ordinary objects are made of. Proximate matter explains how ordinary objects move and change, while also retaining their basic physical identity. In this account of proximate matter Plotinus follows Aristotle to the letter, borrowing Aristotle's instance of gold.Footnote ⁸ What, then are we to say about ultimate matter? Is there a persisting substrate for all things, even after they have been stripped bare, as it were, of all their perceptible properties?

This question brings us to Plotinus's other term for matter, hypodoche, translated ‘receptacle. Plotinus took this metaphor from Plato. Here is a portion of what Plato says about the term:

Plato introduces the obscure and characterless ‘receptacle’, which he also calls space, as the container of bodies, and therefore as extending throughout the physical world. This text along with its attendant doctrine was foremost in the mind of Plotinus when he attempted to clarify his own notion of ultimate matter. What I have called the Stoic plasticine notion, as cited in Text 2, was also influenced by Plato, but with the difference, observed by Plotinus, that the Stoic receptacle is not bodiless but taken to be a completely amorphous body.

Bodiless container, amorphous body – these are very obscure expressions. What philosophical thoughts are driving them? The answer has two related aspects, one I will call idealist and the other realist. The idealist aspect is the notion that ordinary things derive their identity, quality, and quantity from their forms or structures or intelligible natures, expressible in definitions or archetypes, principles that are fully accessible only to the mind. The realist aspect is the evident fact that ordinary objects are composites of a specific form (e.g. cat-form, daffodil-form) and of that in which the individual instance of that form is expressed or manifested – their particular physical and bodily make-up, like the wood of this bed or the gold of this cup. Ordinary objects are not bodiless containers or amorphous bodies, but in order for them to possess the specific identity, quantity and quality that they have, they each require, according to the theories I am discussing, a form-containing constituent entitled matter. The matter, then, is what underlies the form, or what receives the form, and enables the form to have magnitude or physical extension. In this analysis, then, form and matter are essentially correlative notions, presupposing one another, and not instantiated independently of one another. You cannot have form without matter or matter without form.

This co-dependence and correlativity fit Aristotle's and the Stoics' accounts of form and matter. When we study natural objects, we can focus either on what they are made of and on what persists throughout their life or existence, i.e. their material constituents, or on the forms that make them the particular things that they are – a cat or a tree etc., of a certain size, colour, etc. But Platonists such as Plotinus see a need to hugely complicate this analysis of form and matter. The complexity arises because, in their top-down analysis of reality, the forms that make natural objects particular cats or trees etc. are not fully and perfectly present in the objects' feline or arboreal matter. The matter of such objects is what makes them physical repositories of non-physical formative principles. Physical objects in Platonism are material copies of immaterial ideal substances, as David Sedley explained to this Institute in his December 2014 lecture on Plato's Theory of Forms. (You may care to recall Wordsworth's ‘shades of the prison-house’ that turn the ‘light’ of Plato's Forms into ‘the light of common day’ from his famous poem Intimations of Immortality). For Plotinus ultimate matter is an insubstantial substrate and receptacle of immaterial formative principles. In what follows we will see how he struggles to get this meaning across.

4.

In a systematic inventory of the perceptible world Plotinus sets out the following terms:

This list of originally Aristotelian terms would be quite straightforward if Plotinus, like Aristotle, had regarded individual things - this tree, this cat - as the physical world's primary beings or independent substances. Instead, as a Platonist, Plotinus treats physis, his name for the physical world of natural objects, as the domain of only derivative beings. He even calls perceptible objects such as stones, trees and cats, images, the originals of which are the supra-sensible Intelligible Forms. While Plotinus is quite ready to write on occasion as if perceptible things have intrinsic Aristotelian forms and corresponding matter, this is approximate language. The only unequivocal Plotinian beings, are purely incorporeal, intelligible and intelligent entities, objects and activities of divine thought. Perceptible properties and natures are projections into matter that fall short of full reality.

We can now see that Plotinus's two terms for his ‘so-called matter’, substrate and receptacle (Text 1), belong to two different earlier theories. One of these theories, the Aristotelian substrate theory, is well designed to identify the proximate material of natural or manufactured things such as trees or cups (the wood or the gold that underlies such forms) and to account for the primary elements of which wood and gold are formed. Neither proximate matter nor primary elements, however, account for the nature of matter as such, basic physical reality, as it were. The other theory, the Platonic receptacle and its Stoic successor, fulfill this latter role by positing a changeless, spatial container for all physical objects into which they come and go. This theory, however, does not explain the composition of proximate matter, what makes it golden or arboreal, nor does it explain why there are just four primary elements. In addition, neither theory fully faces the question of why there is a physical world, consisting of transient bodies, in the first place.

By synthesizing the substrate and the receptacle theories, Plotinus goes some way to responding to these points. In the process, however, he leaves us with an ultimate matter that is so dark that it is only describable in the negative terms of lack, or privation, or what it is not.

5.

The essay from which I drew Text 5 (Ennead VI.3) concerns the kinds of equivocal being that pertain to the physical world of change and flux. After positing matter as common to all bodies, Plotinus asks whether matter may be regarded as a genus. The Stoics had called their plasticine notion of material substrate ‘primary substance’ [Text 2] and so made it generic and foundational to everything that exists. Plotinus responds that matter cannot be a true genus because it has no essence; hence it does not confer anything substantive on objects. Substance, he argues, must exclusively be a function of form, intelligibility, structure.

May we, then, dispense with ultimate matter, and substitute for it a single genus of ‘perceptible substance’, call it body, that is essential and common to all terrestrial things – stones, earth, water, plants, and animals? The problem with that proposal, Plotinus urges, is that bodies are not uniform – some are complex and organic; others, like the four elements, are ‘more matterish’ (hylikotera), meaning less unitary and less determinate.Footnote ⁹ Body, in other words, is too diverse a notion to constitute the ultimate foundation of all perceptible objects. Ultimate matter on this view must be something more primitive and inchoate than even the simplest body. To get at it, we have to resort to the mental operation of trying to separate all form, even the simplest determination, from embodied things and physical elements.

Plotinus's notion of ultimate matter seems to face two paradoxes, one conceptual and the other ontological. The conceptual paradox is the indispensability of an essentially imperceptible and indescribable substrate to the analysis of perceptible objects. Plotinus explains his position thus (Ennead II.4.5):

To do philosophical justice to this passage, we need to take light and dark quite literally, albeit with reference to mental vision. The obvious aspect or component of a body is its perceptible form. But bodies also have imperceptible depth. How do we know? By analogy with dark as the absence of light, Plotinus infers that bodies have a dark underside, consisting of the absence of what we can actually perceive of their make-up. Ultimate matter, on this account, is essentially something that defies description and perception.Footnote ¹⁰ We know of it only in the way that we know that if we turn off a light, dark supervenes. Dark simply is the absence of light; there is nothing else to it.

Plotinus articulates the ontological paradox in the following passage (Ennead III.6.7. 3):

Bulk or solidity is basic to any notion of body, but whence do bodies derive their bulk? From their matter, surely? But ultimate matter as such has no real bulk or solidity? Ultimate matter is a mere phantom of bulk. How do we deal with this regress? Does it mean that bulk or mass is in some sense illusory, a mere figment of our imagination?

Rather than answer that pressing question now, let me step back and say where I take Plotinus to be coming from in his mystifying statements about ultimate matter. Given his Aristotelian background, might we remove suggestions of paradox by interpreting the two quoted passages as rhetorical rather than doctrinal? As the substrate of the four elementary bodies, Aristotle's ‘prime matter’ is best interpreted as a purely conceptual or logical notion with no directly physical referent. The elementary qualities hot, cold, dry, and wet are basic to the simplest instances of Aristotelian matter that we can experience. There is no such Aristotelian thing as prime or ultimate matter. May we explain Plotinus correspondingly, as he himself sometimes suggests our doing?

Only up to a point, I respond, because ultimate matter, as understood by Plotinus, is absolutely necessary to the nature of the physical world. Ordinary objects, unlike ideal objects, are embodied. Embodiment is a function of ultimate matter as distinct from form. We cannot get at such matter, to examine it in the laboratory, because ultimate matter, as distinct from proximate matter, does not exist as a separable component of things. Yet it furnishes ordinary objects with the basic bodily properties that distinguish them from the ideal objects of thought, viz. spatial extension, changeableness, multiplicity, impermanence, and imperfection.

Plotinus also differs profoundly from Aristotle in crediting ultimate matter with supreme negative value – not as being ‘evil’, as some scholars like to say – but because ultimate matter negates the supreme positive value he attributes to determinate unity, identity, and intelligibility.Footnote ¹¹ What makes ultimate matter bad, illusory, phantomlike, for Plotinus is its complete lack of substantive identity, determinacy, and unity. Its badness is a function of what it lacks.

Plotinus also differs from Aristotle in the way he distinguishes between intelligible, proximate, and ultimate matter. All three of these types of matter satisfy the generic concept of being ‘a substrate and receptacle’, but they fit the concept in the distinct ways that pertain to the three distinct levels of Plotinus's overall metaphysical scheme, to which I now turn.

Intelligible matter, as the substrate of Intelligible Forms, belongs to the level of Nous/Intellect and is therefore unqualifiedly good.Footnote ¹² Proximate matter, as the substrate of perceptible forms, belongs to the level of Soul/Nature. Its value is correspondingly relative to the kind of phenomenal matter that it is, for instance, flesh, bone, bronze. Ultimate matter pertains to the bottom of this scheme, where forms have run out of all claims to substance, leaving only the residue of indeterminate substrate. In his essay on matter, Plotinus presents the topic in exactly this order, starting with the generic concept, and then passing, in turn, to intelligible matter, proximate matter, and finally, and at greatest length, ultimate matter.

Headed by the transcendent One, Plotinus's philosophy involves both a vertical and a horizontal dimension. The horizontal dimension has three aspects, ontological, cognitive, and axiological. The vertical dimension has five levels, ranging from the One down to ultimate matter, with a radical gap dividing the three incorporeal and eternal levels (One, Nous, and Soul) from the corporeal and mortal domain of Nature and the shadowy domain of ultimate matter.

I represent this scheme in the following diagram.Footnote ¹³

According to this hierarchical scheme, ontology, cognition, and value are so related to one another that unity and stability are their common measure at the top and complete privation of stability and determinacy are their common measure at the bottom, as signified by ultimate matter. Plotinus's scheme states, then, that as things or levels become less unitary and stable and determinate, they become less real, less cognitively accessible, and less good. The scheme is an obvious successor to Plato's famous Sun, Line, and Cave analogies (Republic 6–7) but it differs importantly from these prototypes by its explicit incorporation of the principles One, Soul, and Matter. Plato presupposes the existence of bodies, and the inferior status of bodies as perceptible, non-knowable, changeable items, but he does not clearly explain why the world contains them. In Plotinus, Soul, the principle of animate life, creates bodies by generating perceptible images of Intelligible Forms in ultimate, formless matter. To put it another way, ultimate matter is the wall of Plato's cave, distorting true ideas by reflecting imperfect images of them.

The physical world, then, according to Plotinus, is an inferior embodied counterpart to the purely Intelligible and immaterial Forms. This secondary status does not make the physical world unreal or bad; Plotinus takes the physical world to be a trickle-down product of Intelligible Forms and as good as it can be, relative to its derivative status, but its contents, owing to their embodiment and matter, lack the unity and stability of the Intelligible Forms. Particular human souls simultaneously live a life of thought and of sense perception. They (i.e. we) convert apprehension of incorporeal thinkable and thinking realities into the corporeal, perceptible, and animated images that constitute ordinary objects. These objects, like the gold cup or the tree or the cat, lend themselves to analysis in terms of matter and form. But what can we say about matter, when we try to conceptualize it below the level of its proximate vegetable matter or animal matter or metallic matter, or below the level of such matter's constituents, such as the four elements?

6.

Plotinus starts his account of ultimate matter by reaffirming its complete tractability and inherent lack of quality and quantity. Whence, then does matter acquire colour, temperature, weight, size, and shape, which are the basic attributes of bodies Not from itself, evidently, but from

How are we to understand ‘the giver of shape and size’ and ‘the will of the maker’? The answer, supplied by the text, is form, in the sense of ‘formative principle’. But formative principles do not simply float down, as it were, to matter from Nous, the Intelligible level of Plotinian reality. Formative principles [we may perhaps compare DNA, digital information, encoded algorithms] are mediated to matter; they are projected on to it, as I was just saying, by soul, in the sense of World Soul, the providential principle of embodied life. It is tempting to align Plotinus's World Soul and its will with the divine Demiurge of Plato's Timaeus, who imposes mathematical form on the corporeal traces that flit in and out of the ‘receptacle’.Footnote ¹⁴ But this Platonic precedent can only be a loose approximation to what Plotinus is saying here. His ultimate matter is neither a set of corporeal traces nor an independently existing region of empty space. It is something so abstract and amorphous that mind, the mind of the World Soul, can make of it anything it chooses from the stock of Intelligible Forms. Matter only comes into consideration, as we have had occasion to note before, in the light of its correlativity to form.

Next Plotinus asks: ‘How can one grasp an entity that has no size’? His response to this question amplifies the points just made concerning form and soul. Matter independently of form simply lacks size.

To conceptualize absence of size, Plotinus says, we need to invoke ‘the soul's indefiniteness’ (aoristia. Ennead II.4.10). By way of clarification for this expression, Plotinus first repeats his earlier assertion about the way the eye, in the absence of colour, can be said, in a way, to be affected by the dark. But is this really seeing? It can only be so described if the eye sees not nothing, but the absence of everything visible. By analogy,

During normal perceptual consciousness the soul thinks its perceptual objects in terms of their shape, size, and so forth, that is to say, in terms of their perceptible forms and proximate matter, such as the golden cup. The soul can recognize these things because, thanks to its rational faculty, it has the corresponding concepts. But when it comes to thinking of matter as such, ultimate matter, the soul arrives at an impasse, bringing us back to the ontological and conceptual paradoxes I brought up earlier. To continue with Plotinus's own extraordinary words (Ennead II.4.10):

The soul cannot accept a representation of absolute indefiniteness, so it always bestows some form, however minimal, on matter.

Can ultimate matter, then, in view of its obscure and illusory status, be an objective constituent of the physical world? Bodies need bulk or mass (ongkos) to be the recipient of their magnitude and qualities. But since ultimate matter has no magnitude, how can it provide bulk or mass?

Plotinus answers this question by seeming, for the first time, to assign a positive function to ultimate matter. Drawing on his original description of matter as an incorporeal substrate, he says:

What matter receives from forms, it receives in extension, and thus it becomes bulk or mass. How are we to take this claim? Is ultimate matter just empty space? That suggestion could fit its description as pure recipient and substrate. But ultimate matter cannot be empty space in our modern sense of the term, with its objective designation. How, moreover, could empty space endow bodies with mass?

The indefiniteness of ultimate matter gives it its capacity to receive the forms constitutive of bodies. Otherwise, if matter had its own magnitude, it would impose that magnitude on the forms it receives. By its reception of bodily forms ‘matter becomes mass’, or at least we imagine it so. When the forms of things that are naturally embodied encounter ultimate matter, they cause matter to acquire the magnitude appropriate to such bodies. As such, however, ultimate matter is always sheer void – ‘an illusion of mass’ – incapable of being credited with any size of its own. Ultimate matter is a privation, knowable or thinkable by what it is not, and hence something unavoidably dark in a metaphysical scheme that posits the identity of form and intelligibility.Footnote ¹⁵

Rather than shirk this conclusion Plotinus revels in it, as the following dizzying passage exemplifies (Ennead II.5.5):

Plotinian matter is neither nothing nor something. He can call it a phantom, a mirage, a privation, but even these characterizations are meaningful only by courtesy of what matter actually lacks, namely any kind of form or describable or intelligible reality. In light of his Aristotelian inheritance, why didn't Plotinus take ultimate matter to be a purely conceptual substrate, separable only in thought from the primary bodies, the four elements? There are many possible answers to this question. Here are two.

7.

First, Plotinus the Platonist cannot avail himself of the robust Aristotelian notion that embodied beings are basic substances. Embodied things can only be diminished beings, according to him, because they are shadowy reflections of Intelligible Forms. Corporeality is their least substantive attribute. Bodies require matter, in order to be bodies, meaning occupants of space, but this material consistency deprives them of any claim to be unequivocal substances. Rather than underpinning the firm existence of bodies, matter accounts for bodies' impermanence and changeableness, and so prevents their being full-fledged entities.

My second rejoinder is to propose that Plotinian matter brings him quite close to George Berkeley's notorious doctrine that material substance is a self-contradictory notion of which there can be no idea and no knowledge. Recall what Plotinus says concerning the soul's vain attempts to achieve a positive and intelligible representation of matter. What our individual souls can actually think or perceive is the formed aspect, not the material aspect, of bodies. Plotinus presumes that there is more to perceptual things than we can perceive of them, namely their matter, but this is such a strange sense of ‘more’ that it is tantamount, as he repeatedly says, to indefiniteness or ‘not-being’.

The perceptible form/matter compounds that he takes bodies to be are not self-subsistent and independent beings that simply exist in their own right ‘out there’, as it were. They are also, at a prior level, already in the soul, because, as he likes to say, soul is not ‘in’ body, but body is ‘in soul’ (Ennead IV.3.20–2). This dependence of body on soul also goes some way to anticipate Berkeley's rejection of a mind-independent world of material things.Footnote ¹⁶ Plotinus verbally agrees with Aristotle that perceptible objects are composites of matter and form, or substrate and qualities. Yet for Plotinus the matter, taken by itself, is not even a potential being but only a privation – an amorphous receptacle that contributes nothing except receptivity to the forms and qualities constitutive of bodies. Matter as such is a non-entity. That is what is the matter with it.

To return to our modern physicists' dark matter, are we to speculate that what is so called will eventually become clear to the eye of experimental reason? Or may it be the case that science is running out of resources to provide a fully intelligible account of what we have traditionally taken to be out there? If Plotinus has any cognitive relevance to physics, it will not be thanks to a resurgence of Platonic ontology, I suppose, but to the challenge he presents, like Berkeley, to our capacity to conceptualize a completely mindless physical space.Footnote ¹⁷