As far as I know, the explicit term “nondualism” was first used in certain strands of Mahayana Buddhism. I believe it later was adopted by the Vedanta school of Hindu scholastic philosophy. I was fascinated with these as a young man, and was for a time much absorbed in developing a sort of Alan Watts style interpretation of Plotinus’ emphasis on the One as a similar kind of radical nondualism.

Radical nondualism goes beyond the rejection of sharply dualist views like those of Descartes on mind and world, and the different religious dualisms like those of Augustine, the Zoroastrians, the Gnostics, the Manichaeans, or the Samkhya school of Hinduism. Each of these latter has important differences from the others, but what unites them is the strong assertion of some fundamental duality at the heart of things. Radical nondualism aims to consistently reject not only these but any vestige of duality in the basic account of things.

The point of view I would take now is that many useful or arguably necessary distinctions are often formulated in naive, overly blunt ways. We should strive to overcome our naivete and our excessive bluntness, but that does not in any way mean we should try to overcome distinction per se. There can be no meaning — even of the most spiritual sort — without some sort of distinction between things. “All is One” is at best only a half-truth, even if it is a profoundly spiritual one.

Proclus’ Elements

The later neoplatonist Proclus (412-485 CE) was head of the Platonic Academy in Athens, at a time when the Athenian Academy was somewhat notorious as the intellectual center of resistance to the spread of Christianity in the Roman Empire. Nonetheless, his work had a profound influence on the Arabic, Byzantine, and Latin traditions. He is usually cited as the main philosophical influence on the early Christian theologian pseudo-Dionysius, who was taken very seriously by Albert the Great and Thomas Aquinas.

Proclus wrote extensive commentaries on Plato, as well as an influential commentary on book 1 of Euclid’s Elements of Geometry. Hegel called him the greatest dialectician of antiquity. Though I think Hegel by his own principles really should have given that title to Aristotle, Hegel was right to recognize Proclus as important.

Aquinas is credited with recognizing that the Latin Book of Causes — a translation of the Arabic Discourse on the Pure Good — was mostly derived from Proclus’ Elements of Theology. Aquinas treated Proclus himself with considerable respect. Dietrich of Freiberg made significant use of his work, and his student Berthold of Moosburg wrote a very long commentary interpreting the Elements of Theology in Christian terms. The Renaissance theologian Nicolas of Cusa and the maverick Giordano Bruno were much inspired by Proclus.

Along with Spinoza’s Ethics, Proclus’ Elements shares the peculiar distinction of being written in a style visibly influenced by Euclid’s Elements. Euclid’s work has often been cited as a sort of paradigm of demonstrative reasoning. Though Proclus, unlike Spinoza, did not work from explicit definitions and postulates and used a looser style of demonstration, his Elements consists of theorems and a sort of demonstrations.

Proclus defends the neoplatonic idea of a One that transcends being, but as Gwenaëlle Aubry and Laurent Lavaud point out in the introduction to the French collection Relire les Éléments de théologie de Proclus (2021), perhaps his most influential idea is that of a very strong continuity from the highest principles to the most mundane effects, which has been read as a strong assertion of immanence as well as transcendence. He is an important source for all the later theological traditions that want to argue for simultaneous immanence and transcendence.

Proclus very explicitly crystallizes what I have called the generalized “unmoved mover” model of causality in Plotinus. For Proclus, “higher” and “lower” causes cooperate in the constitution of worldly things, but the higher cause is always more of a cause than the lower cause. At the same time, he rejects Plotinus’ identification of matter with evil, while emphasizing all of Plotinus’ more positive affirmations of the goodness of manifestation and the beauty of the cosmos.

In a separate treatise On Providence, he develops a sort of epistemic analogue to the generalized unmoved mover theory. “Providence” (pronoia — literally, “forethought”) is a knowledge-like thing that is superior to knowledge in that it is supposed to be eternal and unextended, and to involve no separation of what we might call subject and object. Proclus develops a subtle and suggestive account of something metaphorically like implicit, unextended “seeds” of forms within the overflowing of the One that transcends all extended form. While the One does not “know” worldly things, it “pre-knows” their unextended “seeds”, within something like what Schelling later paradoxically called the identity of identity and nonidentity.

In the Elements, Proclus argues for an interdependence of being, life, and intellect. While one obvious reading of this would emphasize a foundational role of spiritual beings in Proclus’ metaphysics, I am intrigued that it can also be interpreted as a somewhat “deflationary” account of being, closer to Aristotle, and far removed from later notions of pure abstract existence. We can’t begin to have an account of being, without also having an account of life and intellect. With his endorsement of a One beyond “being”, Proclus had no need for a commitment to a notion of pure “being”.

Bounty of Nature

Nature as we experience it is more characterized by superabundance and diversity of form than by univocal necessity. Even nonorganic phenomena like the weather involve material tendencies toward a kind of dynamic equilibrium. These tendencies — which are even more pronounced with living things — involve an “ability” to spontaneously recover when disturbed, a kind of resilience and adaptability to new circumstances.

The neoplatonists developed a whole metaphysic of “eternal generation” by a kind of overflow. For them, beyond every intelligible essence was something “supra-essential” that could be characterized only indirectly, through its overflowing superabundance. Essence ended up as a kind of after-image of the eternally overflowing primary superabundance of the Good or the One. Transformed in various ways, this notion greatly influenced historical developments in theology, supporting notions of the generosity, providence, and grace of a more personal God.

In a more modest and down-to-earth way, Aristotle had also dwelt on our experience of superabundance, applying it in his biology and in the more general notion of potentiality. In between, the Stoics developed a contrasting emphasis on a univocal direct divine omnipotence with respect to events. In the tradition, all three of these approaches came to be hybridized in all sorts of ways. While I think the approach of Aristotle himself was the best of all, I have a lot more sympathy with theologies of superabundance of form than with theologies of power-over and dominion. (See also Fragility of the Good.)

The One?

Whether or not there is a One, it and the Others are and are not — and appear and do not appear to be — all manner of things in all manner of ways, both in relation to themselves and to each other.

Such was the cryptic conclusion of Plato’s dialogue Parmenides. The Parmenides provides the most extensive and technical example of Plato’s concept of dialectical argument, which revolved around thorough exploration of both sides of binary alternatives. Some early Platonists and a number of modern readers thought it was mainly a logical exercise.

Plotinus followed the lead of the neo-Pythagoreans in treating the One far less equivocally, as the main theological principle. He explicitly identified it with the Good from Plato’s Republic, and made it the source of all things. In the wake of Plotinus, later neoplatonists read the Parmenides as a theological treatise.

Maurice Merleau-Ponty’s 1945 Phenomenology of Perception argued that Plotinus was misguided, and really there is no One. Foucault’s 1969 Archaeology of Knowledge was devoted to questioning presumed unities of all sorts. Other pluralists from Aristotle to William James have made broadly similar arguments.

Talk about whether there “is” a One gets tricky. Plotinus associated Being only with his second principle of intellect (nous), not with the One. The 20th century theologian Paul Tillich gave a nod to this when he suggested that to attribute to God the same existence we attribute to objects in the world should be considered blasphemy. In the middle ages Thomas Aquinas went somewhat in the opposite direction, identifying God with pure Being, and therefore he felt no need for a One above being. But the pure Being Aquinas spoke of was a new and innovative concept that is not the same as any worldly existence, so perhaps the two could be reconciled after all.

“The One” has historically been said in many ways. Usually it does not refer to any sort of entity, but rather to a sort of cosmic fusion in which all things participate; or, more properly, to a pure Platonic form of such fusion, perhaps even the form of something slightly anticipating Kantian unity of apperception. Sometimes on the other hand it seems to be beyond form altogether. Plotinus himself largely invented negative theology, and at one point even said the One was just a conventional name for the utterly ineffable. (See also One, Many; Identity, Isomorphism; Univocity; Theology.)