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.)