The Enneads sets out a layered account of reality descending from a single transcendent source Plotinus calls the One, which is beyond being, beyond description and beyond even the categories of thought, since to describe it in ordinary predicates would already limit what is by definition unlimited. From the One emanates Intellect, nous, the level at which the Platonic Forms exist as eternal objects of contemplation, and from Intellect in turn emanates Soul, which mediates between the eternal, unchanging realm above it and the changing material world below, itself the lowest and least real level of the whole structure, understood by Plotinus not as evil but simply as furthest from the source and therefore least perfect.
Human beings, for Plotinus, are fundamentally souls temporarily embedded in bodies, capable of philosophical and contemplative practice that reverses the descent, ascending back through Soul and Intellect toward a direct, unmediated experience of the One that Plotinus describes, drawing on his own reported experience, as a kind of ecstatic union beyond ordinary language. This emanationist structure, a hierarchy of reality flowing outward from an ineffable source and a corresponding path of return through contemplation, proved extraordinarily influential well beyond pagan philosophy: it shaped Augustine's Christian theology, fed into medieval Jewish and Islamic mysticism including strands of Kabbalah, and remains one of the most thoroughly worked out philosophical accounts of mystical ascent that antiquity produced.