The following is a summary of the Plotinian Ascent to the One as I understand it. I would be happy to receive questions, comments, corrections, etc. (I apologize for its length, which is much greater than I intended.)
The basis of the ascent is the Neoplatonic idea that "like knows like." Therefore, in order to know the One, a person needs to become like it, that is, to acquire the characteristics of the One (unity, impassivity, eternity, luminosity, etc.), as described by Plotinus. Since the One is pure being, this amounts to the exclusion of all aspects of non-being. The actual process described by Plotinus is more structured, for it reverses the order of emanation from the One; therefore the ascent is a sort of reverse cosmogony.
To review, here is an outline of the structure of reality as an emanation from the One, described by Plotinus:
I. The One / the Good / God
II. The Intellectual-principle / Intellect / Spirit / Nous
A. Undifferentiated Love/Desire for the Good
B. Differentiated Intellect / the World of Forms
III. The Soul (the World Soul and individual souls)
A. Intellective Soul (divine life)
B. Reasoning Soul (human life)
C. Unreasoning Soul (animal/plant life)
IV. Matter / Cosmos / World
The Ascent, which is a return to the One, reverses
these steps, and each rung can be seen as a way of making oneself more
like the corresponding level of emanation (which I've added in parentheses
to the rung number, below).
1 (IV). The World. This is the level
of ordinary (nonspiritual) life, governed by the four cardinal (civic,
social) virtues: moderation, fortitude, prudence, justice. Nevertheless,
the soul feels an unconscious love (or desire: Eros) for the Good, which
can prompt it to make the ascent. As Plotinus says, Eros snatches the soul
away (as also in "Cupid and Psyche").
2 (III.C). Control of the Unreasoning Soul. Stages 2-4 involve the purificatory (cathartic) virtues, which concentrate spiritual energy inward and upward, making the soul more like the One and lifting her toward it. Attention is turned toward the spiritual and away from the body and everyday affairs, so that these will not distract the soul. The goal is to calm the lower parts of the soul, which are in contact with the body. (Minor disturbances will be displaced by the influx of spirit.) Pleasures and pains are both distracting, so the philosopher avoids them when possible, and when they occur, attaches no more significance to them than as signals of the condition of the body (which cannot be ignored entirely). Therefore, the philosopher should try to live healthily (diet, sleep, exercise) to avoid disturbances.
Many of the Stoic practices are appropriate here,
for controlling appetites, passions, sensations etc. to achieve a state
of tranquillity. Suffering should not be allowed to affect the soul above
its lowest levels; the higher part should remain inviolate, autonomous
and calm. This is facilitated by premeditation of possible future good
and bad fortune, so that the philosopher is prepared to respond appropriately
3 (III.B). Quieting the Reasoning Soul.
It is important to be clear that the "intellectual" (noetic) processes
at the higher levels are intuitive rather than logical. Plotinus treats
this as a minor step in passing from III.C to III.A, but stopping the inner
discourse can be difficult.
4 (III.A). Awakening the Intellective Soul. This is the part of the individual soul that makes contact with the Intellectual realm (II); it is the immortal, divine part of the soul, but not awakened in most people. This awakening is accomplished by focusing spiritual energy inward and upward, continuing detachment from everyday life and opening oneself to experience divine presence. This is not a process of struggle with oneself (indeed, the will must be set aside), but each step proceeds by coming into the presence of the next higher level, and allowing it to act. Plotinus uses the metaphors of polishing a mirror and of purifying a temple to invite a god. The faculties are turned inward (inner vision, inner hearing, etc.), so as to be open to the divine influx.
In this way the soul discovers its freedom and its independence of the body; it is purified of matter, brought back to its pristine form, and separated from everything that is not its true self. Since the intellective soul survives death, the intent is for it to take the attitude toward the everyday world that it will have after death (the Plotinian version of the philosophical "preparation for death" or "dying before you die").
The lower parts of the soul (III.BC) are calmed
to a state of quiet restfulness; the soul is purified, turned inward, and
concentrated toward its center in order to become simple (like the One).
Continuing practice of these purificatory virtues raises the soul into
the apex of its intellectual part.
5 (II.B). The World of Forms. To understand this rung it is necessary to recall the characteristics of the Intellectual-principle, which is both the divine Intellect and the world of Forms as Ideas in this Intellect. The Forms are living, conscious Ideas in a state of mutual contemplation, which constitutes a living, conscious whole. The whole implies all its parts, and each part implies the whole; the Forms are transparent and interpenetrating. Among the Forms are all the immortal, divine intellects, including the traditional gods and the intellective souls of people. The intellective (noetic) process is more like intuitive flow than logical, reflective thought.
As pure form devoid of matter, the Forms achieve perfection and hence constitute the perfect Beauty of the All moving in its intuitive necessity. The beauty of nature is a reflection of this Ideal Beauty.
In the preceding stages the soul has retained its individual identity, but in this stage its intellective part experiences itself as an integral part of the All. Therefore the soul must set aside the non-being that makes it an individual (by imposing finiteness and separation on it). In this way the soul abandons the individual, particular and contingent, and ascends to the universal and eternal. (This is related to the spiritual exercise known as the View from Above.)
To reach this level, the soul must not think
about the Forms, or even contemplate them as other, but must become them
and experience their organic, fluent mutual contemplation. In this intuitive
flow, the soul is unselfconscious and loses its awareness (as in our everyday
experiences of completely absorbed, competent activity). However, this
state cannot be maintained for long, for the human soul is inherently conscious,
so we soon fall back to a lower level.
6 (II.A). Love of the Good. The object of this stage is to turn away from the Intellect's contemplation of itself and to direct its attention upward, to the Good. To accomplish this it must eliminate form so that it may rise above the multiplicity of the world of Forms and approach the One. That is, the duality inherent in the world of forms must be transcended.
Eros (Love/Desire) leads to something beyond Intellect and draws the intellect to the Good (which has awakened desire), as the lover is drawn to the beloved. Thus the Intellect orbits the One. This Love is superior to the Beauty of the world of Forms, for it is luminous and alive, moving with grace; it is an (unwilled) act of grace by the One. The soul must ignore everything but the luminous energy of this love.
By transcending the duality of thought (even of intuition) the calm, sober Intellect extends beyond itself and enters a state experienced as inebriation: drunk on divine nectar, or on the love of the Good. The soul, through identification with the Intellect, comes into immediate contact with the Good, although they are still two (as subject and object, or lover and beloved). This is experienced as voluptas, the happiness, bliss and joy of love.
This stage appears suddenly and usually does not
last for long, since it can be maintained only so long as one doesn't become
conscious of it.
7 (I). Union with the One. Obliteration of the duality of subject and object. The ecstasy of lover joined with beloved, desire fulfilled. Literal enthusiasm (the soul filled with the divine). An experience of peace, gentle love, certainty, well-being, ineffable delight.
Since the One is devoid of activity, form and thought,
the philosopher must abandon activity, form and thought to identify with
the One. Then center will be joined with center. However, neither this
experience nor the preceding ones can be brought about by will; it is a
kind of grace. One can only prepare and wait patiently.
The preceding is the inward approach; Plotinus also suggests an outward approach, which differs in stages 2 and 3:
2'. By contemplating the beauty of the sensible world (IV), the philosopher discovers the World Soul (III) above it, and hence by analogy the superiority of the human soul to its body.
3'. The philosopher discovers that the World Soul
(III) derives its power from contemplating the Intellect and World of Forms
(II). So also, he or she discovers, the human soul receives the light of
Return to the World
The goal of the philosophical life is perpetual attention to the divine, a metamorphosis or deification. However, while still attached to a human body, the philosopher cannot maintain the concentration and tension of union, so he or she must return to the world.
Nevertheless, as a result of union the philosopher returns with improved mildness, gentleness and receptivity, and with an improved knowledge of the true self. He or she is better able to see the light of the One reflected in the everyday world and, by bringing that light back, to illuminate others.
Having experienced the divine union, the philosopher is anxious to return to it, and prepares for it by exercising the virtues and by other spiritual practices. Therefore the philosophical life is amphibious: a double life, cycling, turning now toward the light, now toward the earth.
As some Neoplatonist said, the philosopher is like the moon in its orbit, turning its face first to the earth (when it's full), then to the sun (when it's new). When the moon approaches closest to the sun, when they are in conjunction, there is an eclipse; that is like the philosopher's union with the source of light, when the earthly realms are left in utter darkness. But when the philosopher returns, and turns his face again to the earth, then he illuminates it with light reflected from the source.