Plotinus (204-270 CE) was one of the founders of Neoplatonism, meaning he began his philosophy from his reading and interpretation of Plato. Plotinus asserts that everything emanates from the One, and the concept of beauty has its ultimate origin in the One. It’s tempting to think of the One as God, but despite some theological language, this is not exactly what Plotinus means. The One is the source of everything because there must be something simple from which all composed things derive. The important thing for our purposes is that the One is, in this sense, the cause of beauty.
Plotinus’s writings were preserved by his student Porphyry and assembled into their current form as The Enneads. This work consists of 54 treatises arranged in six sections of nine.