Not to be absolutely certain is, I think, one of the essential things in rationality.Bertrand Russell
Adventure upon all the tickets in the lottery, and you lose for certain; and the greater the number of your tickets the nearer your approach to this certainty.Adam Smith
Hope is not the conviction that something will turn out well but the certainty that something makes sense, regardless of how it turns out.Vaclav Havel
There is only one corner of the universe you can be certain of improving, and that's your own self.Aldous Huxley
Mothers are fonder than fathers of their children because they are more certain they are their own.Aristotle
The whole problem with the world is that fools and fanatics are so certain of themselves and wiser people are so full of doubts.Bertrand Russell