The soul approaches union not by adding something but by removing: stripping away multiplicity, desire, even the activity of intellectual contemplation that is the highest ordinary state of the soul. At the level of Intellect, there is still a distinction between the thinker and what is thought — a residual duality that, however refined, keeps the soul at one remove from the absolutely simple. To approach the One, even this duality must be relinquished. The soul must become as simple as the One itself: not by suppressing its activity through willpower but by finding that, in the deepest interiority, simplicity is already what it is.
What happens in the moment of union cannot be reported in its occurrence — there is no subject there to record it. Plotinus describes it only after the fact: a sudden simplicity, a stillness, an identity with something that was always present but always missed because sought outside. The soul does not become the One in the sense of absorbing it or merging with it as one thing absorbs another; rather, the soul discovers that the principle it was seeking was, in some sense that cannot be made fully explicit, what the soul most fundamentally is. Porphyry records that Plotinus achieved this union four times during the years of their acquaintance.
Union cannot be sustained: the soul returns to multiplicity, to embodiment, to the ordinary concerns of a life that has not ceased to be a life in the world. But the return is not a loss. The soul that has touched the One returns knowing what it has touched — knowing it with a certainty that no argument could produce and no further argument could displace. This knowledge reshapes everything: the philosopher's relationship to beauty, to other people, to death, to the body changes when the point from which one looks at them has been, even briefly, the One itself. The life of philosophy is the attempt to make that brief contact more and more the orientation of an entire existence.
Henosis is the culminating theme of Ennead VI.9. Porphyry's account of Plotinus's mystical experiences appears in the Life of Plotinus §23. The concept of mystical union profoundly influenced Augustine, the Pseudo-Dionysius, and the entire Christian contemplative tradition.
