III 6: The Impassivity of the Unembodied
Consider a mirror: it reflects the image of other objects, but it is in no way affected by doing so. The reflected image comes from the mirror, and not from outside of it, and may be colored by the qualities and fineness of the mirror.
When we spoke of Matter earlier, this is why we called it a reflection: the Ideal does not break apart and enter into Matter, but rather Matter reflects the Ideal, giving rise to the corporeal images we see, and this reflection is the best one possible given the nature of Matter. But Matter itself is not affected: it is permanent and changeless; it is the images reflected in matter that are ceaselessly changing form, in the same way that the reflected image in a mirror may move while the mirror itself is stationary.
This is the same as how transient feelings of the body may be reflected in the eternal soul. For example, suppose one is looking at a statue. The image of that statue is transmitted to the eye and stimulates the body, but this stimulation comes from within the body itself: no piece of the statue enters the body. The body therefore reflects the image of the statue. In the same way in turn, the body stimulates the soul: the mind's image of the statue is formed within itself out of it's own mind-stuff by observing the body. The soul therefore reflects the image of the body's reflected image of the statue. No piece of the body is transported to the soul, and certainly no piece of the statue is, either—the soul already had the capacity for apprehending the image of the statue within it.
This tractate clarifies for me Plotinus' stance on evil. It is like seeing a woman in a mirror: the mirror may reflect her beauty, but that doesn't make the mirror itself beautiful. In the same way, Matter is "evil" in the sense that it reflects the Good, but is not itself Good. He discusses this in §11.
I also like how this makes sense of the paradoxical feeling I've always had that the world we see with our eyes is simultaneously solid on the one hand and illusory in the other. It's like how if you hold up a black piece of paper to a mirror—temporarily halting it from reflecting light—you can see scratches or whatever in the mirror's surface. Similarly, if you could somehow take away everything that Matter reflects, then you'd be able to peel back the illusion and see Matter itself! Just one problem: there'd be no you left to see, hence there's no way to peel back the illusion (from this side of the mirror, at least), and so it has the appearance of reality. Plotinus discusses this in §13.