Full Idea
When Protagoras quipped that man is the measure of all things, he had in mind, of course, the knowing or perceiving man. The grounds are that they have perception/knowledge, and these are said to be the measures of objects. Utter nonsense!
Gist of Idea
Protagoras absurdly thought that the knowing or perceiving man is 'the measure of all things'
Source
comment on Protagoras (fragments/reports [c.441 BCE]) by Aristotle - Metaphysics 1053b
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.289