Plotinus opens On the Beautiful by challenging the standard ancient account that beauty consists in well-proportioned parts. If beauty were symmetry, then only composite things could be beautiful — but a simple colour, a single tone, the light of the sun, each strikes us as beautiful without being a proportion of anything. Something else must be at work. That something, Plotinus argues, is Form: the presence of an intelligible principle that organises matter and shines through it. Beauty is not in the arrangement of parts but in the principle that makes the arrangement cohere.
At the summit of Plotinus's hierarchy of beauty stands the Good — the One itself — which is the source and measure of all beauty below it. Intellect is beautiful because it contemplates the One; the Forms are beautiful because they are the content of Intellect; souls are beautiful when they are ordered by virtue and turned toward Intellect; bodies are beautiful when their matter is shaped by Form. The Good does not have beauty as one of its attributes — it is beauty in the primary sense, the standard against which all other beauty is measured. To love the beautiful truly is to love the Good.
The perception of beauty is never merely aesthetic for Plotinus: it is always also an ethical summons. To recognise beauty is to recognise something as more real, more perfect than what one currently possesses. The soul that genuinely perceives beauty in a body is not invited to rest in that body but to follow the beauty upward — to ask what kind of principle could shine so clearly through perishable matter, and then to seek that principle in its purer forms. Beauty in the world is an invitation to ascent; the philosopher is the one who accepts that invitation fully rather than stopping at the first beautiful thing encountered.
The identification of the Good and Beauty is developed in On the Beautiful (Ennead I.6) and taken up again in the later Ennead V.8 ("On Intelligible Beauty"). The argument against symmetry appears in I.6.1.