"The heart of a fool is in his mouth, but the mouth of a wise man is in his heart." 

-Benjamin Franklin

"You can cheat an honest man but you can not make a fool out of him." 
-Confucius

"A fool is one who goes on trusting;
a fool is one who goes on trusting against all his experience. You deceive him, and he trusts you;
and you deceive him again, and he trusts you;
and you deceive him again, and he trusts you.
Then you will say that he is a fool, he does not learn. His trust is tremendous; his trust is so pure that nobody can corrupt it. Be a fool in the Taoist sense, in the Zen sense. Don't try to create a wall of knowledge around you. Whatsoever experience comes to you, let it happen, and then go on dropping it. Go on cleaning your mind continuously; go on dying to the past so you remain in the present here-now, as if just born, just a babe." 

-Osho