Full Idea
The Sceptics say that they only employ reason as an instrument, because it is impossible to overturn the authority of reason, without employing reason.
Gist of Idea
Sceptics say reason is only an instrument, because reason can only be attacked with reason
Source
report of Pyrrho (reports [c.325 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 09.Py.8
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.408