He probably considered himself as an epigone who is trying to renew Plato’s philosophy. He is the founder of Neoplatonism, but one should not forget that this term dates from the eighteenth century. We know of his life mostly through the biography written by his student Porphyry (c. 234–310) who edited Plotin's manuscripts. This also included the main text, the Enneads. (Plotinus himself never published anything).
The Enneads comprise six volumes, each having nine (ennea, hence the name) sections. They are classified according to a thematic ascension starting with ethics (Enneads, I) then moving on to physics (II), to cosmology (III), to the soul (IV), the intellect (V), and finally to Being and the One (VI).
Porphyry tells us that Plotinus was born in Egypt and studied Platonism in Alexandria. In 247 he moved to Rome in order to teach philosophy. Plotin’s philosophy is based on his reading of Plato, which was influenced by his reading of Aristotle, but also to religious sources that are harder to identify. His reading of Plato has continued to influence our understanding of Platonism. Neoplatonism, by way of Saint Augustine and Boethius, left a mark on Christian dogmatics and thereby influenced all medieval and modern philosophy. Plotinus read Plato as a thinker of the One from which all things proceed and to which all things aspire to return—a procession and return (even a conversion) to the One that beats the rhythm of the universe’s cosmic drama. Although the One is the principle of all things, it lies beyond Being. Plotinus thus takes the radical transcendence of Plato’s first principle, the "epekaina tes ousias," quite literally. For a lack of a better word, he calls this principle the One, but he also calls it the Good, the First, or the Divine. Indeed he uses the term for lack of a better word since the One can be neither expressed nor apprehended. All that can be said of it is what it is not. Plotinus is thus the founder of what will be called, much later, negative theology. Plotinus’s metaphysics is rigorous and mystical. It is rigorous because it endeavors to show how everything, including Being itself, proceeds from the One, but it is also mystical since it appeals to a vision of the One to which the soul can be united in an ecstatic experience.
The main strategy of Platonic thinking is the combination of explanation and reductionism. The complexity of the world is derived from simple principles or axioms. That is, ultimate explanations of phenomena and of contingent entities can only rest in what itself requires no explanation. If what is actually sought is the explanation for something that is in one way or another complex, what grounds the explanation will be simple relative to the observed complexity. Thus, what works as an explanation must be different from the sorts of things explained by it. According to this line of reasoning, explanantia that are themselves complex, perhaps in some way different from the sort of complexity of the explananda, will be in need of other types of explanation. In addition, a multitude of explanatory principles will themselves be in need of explanation. Taken to its logical conclusion, the explanatory path must finally lead to that which is unique and absolutely uncomplex.
The One is such a principle. Plotinus found it in Plato’s Republic where it is known as "the Idea of the Good."
Plotinus has a strong influence on Christianity and on Mysticism.
He calls the source of everything "the One," the principle of unity, something that cannot be named any further (negative theology.) Being One is not a predicate. It is One. No matter what we say, we always say something. Strictly speaking, the One does not have a name. To say that it is one is really a makeshift solution, a manner of speaking.
The idea of the "One," is not in the thing itself. We achieve it through a process of reductive reflection: Nothing can be if it is not one. There is no army if it is not one, nor any chorus if it is not one. Plotinus here refers to the last hypothesis of Plato’s Parmenides (166 c): “if there is no one, there is nothing.” This hypothesis is the foundation of Neoplatonism. (See Grondin, Jean: p. 70).
According to Plotinus, the principle of Being cannot belong to the domain of Being itself. Being is therefore derived from the One, a derivation akin to a “procession,” which Plotinus explains with his famous three “hypostases.”
Plotinus distinguishes three stages: The One, the intellect, and the soul. (this construction precedes the idea of Christian Trinity, formalized in the First Council of Nicaea, 325.)
According to Plotinus, the principle of Being cannot belong to the domain of Being itself. Being is therefore derived from the One, a derivation akin to a “procession,” which Plotinus explains with his famous three “hypostases.”
As it is first, the One is necessarily perfect. As perfection, it “super-abounds” and overflows into a form of “existence,” thus creating an internal difference to itself. Being is born out of the One’s extreme abundance. But since it proceeds from it, this newly begotten thing, which still bears a trace of its origin, returns (epistrophè) or flows back to the One.
Plotinus calls this vision in being, directed back at the One, intelligence or nous. Intelligence and Being thus constitute the One’s second “hypostasis.”
Plotinus differs from Aristotle: Intelligence is not the principle of reality since it presupposes vision and something seen, and therefore a duality. Unity is always first. The One thus suffuses Being and intelligence since what is seen, or made intelligible, is always the One and Being as its emanation.
Then, continuing the work of the One, the second hypostasis, or existence, flows outward and gives birth to another unity, the third hypostasis, namely, the soul.
The layering of this generative process of overflowing produces the world, hypostasis after hypostasis.
There are several ways of speaking of the One. Each of the Enneads is a way that leads to it or follows from it. The self-sufficient One has no need of us to know what it is.
All discourse about the One is therefore a discourse of the soul about itself and its longing to return to the One. Plotinus writes: "For to say that it is the cause is not to predicate something incidental of it, but of us, because we have something from it while that One is in itself; but one who speaks precisely should not say “that” or “is”; but we run round it outside, in a way, and want to explain our own experiences of it, sometimes near it and sometimes falling away in our perplexities about it." (Enneads, VI, 9, 3.)
Despite the impossibility of expressing or understanding it with words, we may still grasp it since we ourselves proceed from it. In Platonic terms, we are always bathed in its light. We can feel its presence and be reunited with it. For Plotinus, love best expresses this union: love desires a union with the one that would make us whole.
The soul, which proceeds from the One, seeks in vain to exist by itself and to be, like the One, a self-sufficient principle. This constitutes the vanity of all souls carried away from the One as if by some centrifugal force. Plotinus’s philosophy tries to draw the soul back from its estrangement, from its separation in time, and return it to its essential center. He calls on us to convert and to return by following in reverse order the procession of things back to the One. The road is long and solitary, and this situation expresses the essential, perhaps tragic, tendency of the human soul. The last line of the treatise, which Porphyry placed at the end of all the Enneads and which must have appeared to him as Plotinus’s final words: “the passing of solitary to solitary” [Enneads, VI, 9].
It is easy to see how the Christian Church Fathers recognized the Neoplatonic vocabulary as exteemely useful for them in terms of expressing the Christian faith. They borrowed Neoplatonic terminology in the development of the doctrine of the three hypostases of the Trinity. The procession from the One through over-abundance can also be used to explain the idea of "creatio ex nihilo," God as the only principle of creation for the world from nothing. This idea has no equivalent in Plato or Aristotle’s metaphysics. Since the generation of Being was the result of the superabundance or generosity of the first principle, it was then easy for Christians to recast it as love and light.
Neoplatonists, however, would have great difficulties accepting the Christian belief of God's incarnation in the flesh.
A commentary on Plato’s Parmenides by Porphyry contains the first occurrence in Western thought of the “ontological difference” between Being and beings.