Considered the last of the great pagan philosophies, it was developed by Plotinus (3d cent. A.D.). It has had a lasting influence on Western metaphysics and mysticism, although its original form was much altered by the followers of Plotinus. Neoplatonism was a viable force from the middle of the 3d cent. to 529, when Justinian closed the Academy at Athens. Although Plotinus is the central figure of Neoplatonism, his teacher, Ammonius Saccus (175–242), a self-taught laborer of Alexandria, may have been the actual founder; however, no writings of Ammonius have survived. Plotinus left Egypt, settled in Rome in 244, and founded a school there.
The enduring source of Neoplatonist thought is the Enneads of Plotinus, which were collected and published after his death by his student Porphyry, a Phoenician. Plotinus' purpose was to put into systematic form an idealistic philosophy and thus combat the trends of Stoicism and skepticism that had crept into interpretations of the philosophy of Plato. Plotinus rejected the dualism of two disparate realms of being (good and evil, material and transcendent, universal and particular) and set forth instead one vast order containing all the various levels and kinds of existence.
At the center of the order is the One, an incomprehensible, all-sufficient unity. By the process of emanation the One gives rise to the Divine Mind or Logos [word], which contains all the forms, or living intelligences, of individuals. The content of the Divine Mind, therefore, constitutes a multiple reflection of the unitary perfection of the One. Below the divine mind is the World Soul, which links the intellectual and material worlds. These three transcendent realities, or hypostases (the One, the Divine Mind, and the World Soul) support the finite and visible world, which includes individuals and matter. Plotinus sometimes compared the One to a fountain, from which overflowed the lower levels of reality.
The Neoplatonic cosmology also had religious overtones, for Plotinus believed that people potentially sought a life in which the individual soul would rise through contemplation to the level of intelligence (the Divine Mind) and then through mystic union would be absorbed in the One itself. Conversely, a privation of being or lack of desire toward the One was the cause of sin, which was held to be a negative quality (i.e., nonparticipation in the perfection of the One). There are thus two reciprocal movements in Neoplatonism: the metaphysical movement of emanation from the One, and the ethical or religious movement of reflective return to the One through contemplation of the forms of the Divine Mind.
While Plotinus' thought was mystical (i.e., concerned with the infinite and invisible within the finite and visible world), his method was thoroughly rational, stemming from the logical and humanistic traditions of Greece. Many of his philosophical elements came from earlier philosophies; the existence of the One and the attendant theory of ideas were aspects of the later writings of Plato, particularly the Timaeus, and Stoicism had identified the World Soul with transcendent universal reason. What was distinctive in Plotinus' system was the unified, hierarchical structuring of these elements and the theory of emanation.