Full Idea
Sceptics say that every demonstration depends on things which demonstrates themselves, or on things which can't be demonstrated.
Gist of Idea
Sceptics say demonstration depends on self-demonstrating things, or indemonstrable things
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.413
A Reaction
This refers to two parts of Agrippa's Trilemma (the third being that demonstration could go on forever). He makes the first option sound very rationalist, rather than experiential.