Full Idea
He began one of his books as follows: 'Man is the measure of all things - of the things that are, that they are, and of the things that are not, that they are not'.
Gist of Idea
Man is the measure of all things - of things that are, and of things that are not
Source
Protagoras (fragments/reports [c.441 BCE], B01), quoted by Diogenes Laertius - Lives of Eminent Philosophers 09.51
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.211