Earth cares for her own ruins, naught for ours. Nothing is certain, only the certain spring.

-Binyon, (Robert) Laurence
  'The Burning of the Leaves'.

Sincerity is all that counts is a widespread modern heresy. Think again. Bolsheviks are sincere. Fascists are sincere.Lunaticsaresincere.Peoplewho believethatthe earth is flat are sincere. They can't all be right. Better Sir Martin Mar-All make certain first that you have something to be sincere about, and with.

-Bradwell
  In the Daily Express.

As faras the laws of mathematics refer to reality, theyare not certain, and as far as they are certain, they do not refer to reality.

-Einstein, Albert
Quoted in Fritjof Capra The Tao of Physics (1975), ch.2.

In this world nothing can be said to be certain, except death and taxes. See Defoe 258:25.

-Frank, Anne
  Letter to  Jean Baptiste Le Roy,13 Nov.

Delegimus certum otium studiorum, quam incertum negotium bellorum. We have opted for the certain leisure of study, rather than the uncertain business of war.

-Gerbert later Pope Sylvester II
  Letter to Monk Raymond.

Certum est quia impossibile est. It is certain because it is impossible.

-Tertullian full name  Quintus SeptimiusFlorensTertullianus
De Carne Christi, ch.5.The phrase is commonly rendered'It is so extraordinary that it must be true.'