Full Idea
No purer metaphysical doctrine can possibly be found than the Protagorean thesis that to be (anything at all) is to be relative ( to something or other).
Gist of Idea
There is no more purely metaphysical doctrine than Protagorean relativism
Source
comment on Protagoras (fragments/reports [c.441 BCE]) by José A. Benardete - Metaphysics: the logical approach Ch.3
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.16