The Stoa might be not the first philosophical school that comes to mind when considering the most important ancient contributions to aesthetics, yet multiple extant fragments show that the Stoics had a non-marginal theoretical interest in aesthetic properties. Probably the most important piece of evidence for the Stoic attempts to theorize beauty is the definition of beauty as summetria of parts with each other and with the whole. In the first half of this article, I present and analyse the main (...) evidence for this definition. Then I discuss Plotinus' critique of the definition and argue that it contains some pertinent remarks that, with support of additional evidence, lead to the conclusion that the Stoics conceptualized aesthetic properties as supervening on functional composition. (shrink)