The Argument from Relativity, AR, is not applied to the beautiful in the Phaedo. It stands behind the argument of 74a–c, since Socrates thinks AR can be applied to any of a range of terms including the beautiful (76d8, 78d3). The Argument from Relativity does not appear in the Symposium, but plainly stands behind Diotima's description of the beautiful in 210e–211a.

DIOTIMA'S IMMORTALITY

According to Diotima, a human being can only find immortality by leaving behind “another new one such as it itself was” (208a7–b4). Some, according to her, do this bodily, by begetting human offspring (208e); others do it in soul (208e–209a): they are pregnant with “wisdom {φρόνησις} and excellence in general” (209a3–4), and when they come of age, they desire to beget these things (209ab). They seek out beautiful bodies, but prefer the combination of these with beautiful souls as well (209b); with such people they beget poems, as did Homer and Hesiod (209d1–3), and laws, as did Lycurgus and Solon (d4–e2): these win fame for “begetting every sort of excellence” (γεννήσαντες παντοίαν ʾɑρετήν, e2–3).

This much, she says, even Socrates might be able to grasp (209e5–210a1); she doubts whether he is up to the “final and highest mysteries for the sake of which these exist in the first place,” but she is prepared to have a shot at getting them across (210a1–4).