"The activities of nerve cells do not reflect an environment independent of the living organism and hence do not allow for the construction of an absolutely existing external world." --Humberto Maturana (M)
Do not let what you know, or what you think you know, get in the way of what you see right in front of your eyes.

Of course there is something to be said for the other side of this notion as well: Do not let casual observation cloud the clarity that can be achieved by a wise combination of what one knows and what one sees. (M)