Full Idea
Sceptics actually assert two dogmas: that nothing should be defined, and that every argument has an opposite argument.
Gist of Idea
Scepticism has two dogmas: that nothing is definable, and every argument has an opposite argument
Source
Diogenes Laertius (Lives of Eminent Philosophers [c.250], 9.Py.11)
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.418