Full Idea
Prodicus insists that contradiction is impossible, since if two people are contradicting each other, they cannot both be speaking of the same fact. Only the one who is speaking the truth is speaking of facts as they are; the other does not speak facts.
Gist of Idea
Contradiction is impossible, since only one side of the argument refers to the true facts
Source
report of Prodicus (fragments/reports [c.423 BCE]) by Didymus the Blind - Commentary on Ecclesiastes (frags)
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.246
A Reaction
cf. Kant's 100 thalers example