There is this quote painted on a buildings facade in the Stoopstraat in Antwerp. It is quite visible even from the bicycle and it is one of those things that makes you think while you pass by, specially when you are stopped by red light. Although according to the wall the quote originates from Frederik van Eeden, other sources attribute it to another Dutch writer, Multatuli.

Or "Nothing can be known, not even this" [Carneades, academic scepticism].