memory follows upon attention; those who have memorized much, by dint of their training in the use of leading indications [suggestive words and the like], reach the point of being easily able to retain without such aid: must we not conclude that the basis of memory is the soul-power brought to full strength? The lingering imprints of the other explanation would tell of weakness rather than power; for to take imprint easily is to be yielding. An impression is something received passively; the strongest memory, then, would go with the least active nature. But what happens is the very reverse: in no pursuit to technical exercises tend to make a man less the master of his acts and states. It is as with sense-perception; the advantage is not to the weak, the weak eye for example, but to that which has the fullest power towards its exercise. In the old, it is significant, the senses are dulled and so is the memory. Sensation and memory, then, are not passivity but power.
— Plotinus
Plotinus is pressing against something we still assume: that to be impressionable is to be sensitive, and that sensitivity is depth. The yielding wax takes the mark most clearly — so the metaphor goes, from Plato forward — and we have learned to read that yielding as a virtue of the interior life, proof that the soul is open, receptive, present. Plotinus refuses it. The mind that holds everything it has met does so not because it is soft but because it has gathered force toward its own exercise. Memory, like sight, belongs to power, not passivity.
What this means for the spiritual argument is worth sitting with. Much of what gets called interiority — receptivity, openness, letting go of control — carries a pneumatic promise underneath it: if I am surrendered enough, I will not have to struggle. Plotinus knows better, and he is no enemy of ascent. His point is structural: the soul that can actually attend, retain, and return to what it has experienced is not the one dissolved into pure receptivity, but the one that has built something in itself. The impression is received, yes — but receiving is not the achievement. The achievement is what the soul does with received content when its own capacity is at full strength.
Plotinus·The Six Enneads·270