Full Idea
I know well enough what time is, provided that nobody asks me; but if I am asked what it is and try to explain, I am baffled.
Gist of Idea
I know what time is, until someone asks me to explain it
Source
Augustine (Confessions [c.398], XI.14)
Book Reference
Augustine: 'Confessions', ed/tr. Pine-Coffin,R.S. [Penguin 1961], p.264
A Reaction
A justly famous remark, even though it adds nothing to our knowledge of time. This sort of thought pushes us towards accepting many things as axiomatic, such as time, space, identity, persons, mind.