A main channel through whom Plato's ideas influenced the middle ages, Plotinus (204-270 CE) and his disciple Porphyry combined Plato's rationalism with mysticism to produce a powerfully influential version of neo-Platonism. Plotinus' works were edited and collected by Porphyry into six books of nine chapters each, known as the Enneads (Greek for "The Nines").
Plato had suggested, in Book VI of the Republic, that the Form of the Good was supreme in the world of the Forms. In fact, he said,
What gives truth to the objects of knowledge, and to the knowing mind the power to know, is the Form of the Good. As it is the cause of knowledge and truth, think of it also as being the object of knowledge. Both knowledge and truth are beautiful, but you will be right to think of the Good as other and more beautiful than they. As in the visible world light and sight are rightly considered sunlike, but it is wrong to think of them as the sun, so here it is right to think of knowledge and truth as Good-like, but wrong to think of either as the Good, for the Good must be honored even more than they. ... [A]s for the objects of knowledge, not only is their being known due to the Good, but also their real being, though the Good is not being but superior to and beyond being in dignity and power" (Republic 508e-509c).
Plotinus' philosophy may be seen as a set of variations on this Platonic theme. He refers to the Supreme Form more frequently as The One than as The Good, and emphasizes its aspects of Unity, Intelligence, and Soul or Life. Everything that is emantes from the One, and is drawn back toward it. For humans, the "flight of the alone to the alone," as Plotinus called it, is marked by rational inquiry (since the forms are rational and known by the rational mind). but it is also marked by a mystical experience which transcends reason, as the soul goes into itself and returns to Unity with its source. This element of mysticism in Plotinus was a major source of inspiration for medieval Christian mystics and theologians. Recent scholars like Jules Brehier have speculated that Plotinus may have been influenced by direct acquaintance with Hindu mysticism. Whether he was or not, his mystical teachings are certainly similar to those of the Upanisads, and seem to reflect a similar mystical experience.
The Enneads contain a chapter on Beauty (I.6) which was highly influential in the Middle Ages. After considering other theories of what beauty is, Plotinus concludes that it is formal Unity. When diverse or similar parts are unified by one form, the Soul recognizes and takes pleasure in the form of Unity. This may happen when we view a painting or a sculpture, listen to a piece of music, or follow an elegant mathematical proof. In all these cases, we are drawn toward Unity, and the form of Beauty Itself. We must get there by stages: like people emerging from a dark cave into sunlight, we must become accustomed to the light. In the following passage, Plotinus combines ideas from Plato's allegory of the Cave with themes from the Symposium:
Like anyone just awakened the soul cannot look at bright objects. It must be persuaded to look first at beautiful habits, then the works of beauty produced not by craftsmen's skill but by the virtue of men known for their goodness, then the souls of those known for beautiful deeds . . . Only the mind's eye can contemplate this mighty beauty . . . So ascending, the soul will come to Mind . . . and to the intelligible realm where Beauty dwells (Enneads I.6.9).
Bibliography: John Haldane, entry in A Companion to Aesthetics, ed. David Cooper, Blackwell, 1992, 1995