BLUMENTHAL, H.J.. Plotinus' Psychology. The Hague: Martinus Nijhoff, 1971, p.6.
But it is not possible to treat in isolation the picture of man as a part of the sensible world, occupying a fixed place in the structure of reality. No form of being, in Plotinus' philosophy, is cut off from that above it. So Noûs remains connected with the One, Soul, as a hypostasis, with Noûs, and the individual soul with all soul. Our soul does not descend completely (IV.8.8.2f., V.I.10.13-18, VI.2.22.31-3), but a part stays up in the intelligible world. This is the main cause of Plotinus' difficulties in answering the question “who are we?”. Two consequences of the view that the intellect is always transcendent led to its abandonment by most later Neoplatonists. If the highest part of our soul remains above, we need to explain how it comes about that we do not always think (noein). Proclus, who maintained that if the higher part of the soul always thought it would be an entity of a different kind from the rest of the soul, argued that if it thought intermittently there would be a single substance composed of what always thinks and what sometimes thinks; this was impossible, so the soul must descend as a whole. The difficulty about perpetual intellection involved another, and Iamblichus and Proclus also found it necessary to detach the soul completely from the higher world to explain its imperfection. If the highest part of the soul is perfect, as a constantly thinking noûs would be, then the whole must be perfect and happy, which clearly it is not.