Full Idea
It is a fundamental principle of dialectic that every statement is either true or false. So is this a true proposition or a false one: "If you say that you are lying and say it truly, you lie"?
Gist of Idea
Dialectic assumes that all statements are either true or false, but self-referential paradoxes are a big problem
Source
M. Tullius Cicero (Academica [c.45 BCE], II.xxix.95)
Book Reference
Cicero: 'De Natura Deorum and Academica (XIX)', ed/tr. Rackham,H. [Harvard Loeb 1933], p.587