Full Idea
When sceptics say that they define nothing, and that every argument has an opposite argument, they here give a positive definition, and assert a positive dogma.
Gist of Idea
When sceptics say that nothing is definable, or all arguments have an opposite, they are being dogmatic
Source
Diogenes Laertius (Lives of Eminent Philosophers [c.250], 9.11.11)
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.418