It is certain because it is possible. Tertullian
An angel fell from Heaven without any other passion except pride, and so we may ask whether it is possible to ascend to Heaven by humility alone, without any other of the virtues. John Climacus
It is possible to build a virtual-reality generator whose repertoire includes every possible environment. David Deutsch
It is possible for a ram to kill a butcher. Danish Proverb
Never fight an enemy whilst it is possible to cheat him. German Proverb
It is impossible to find a man who has everything, but it is possible tofind one who enjoys the things he has. Maasai Proverb