“Even if it's absurd to think you can change things, it's even more absurd to believe that it is foolish and unimportant to try.”

Preface (p. xiii; quoting Voltaire)
Irreligion: A Mathematician Explains Why the Arguments for God Just Don’t Add Up (2008)

Source: 11/22/63

Source: Around the Day in Eighty Worlds

Patheos, The Cow http://www.patheos.com/blogs/reasonadvocates/2016/01/22/the-cow/ (January 22, 2016)

Source: Birdy

Variant translations
It is by all means to be believed, because it is absurd.
It is is entirely credible, because it is inept.
It is certain because it is impossible.
De Carne Christi 5.4
Often paraphrased or misquoted as "Credo quia absurdum."
Also paraphrased as "It is so extraordinary that it must be true."
Two lines from De Carne Christi have often become conflated into the statement: "Credo quia impossibile" (I believe it because it is impossible), which can be perceived as a distortion of the actual arguments that Tertullian was making.

Reflections on a chestnut tree root.
Nausea (1938)
Context: Absurd, irreducible; nothing — not even a profound and secret delirium of nature — could explain it. Obviously I did not know everything, I had not seen the seeds sprout, or the tree grow. But faced with this great wrinkled paw, neither ignorance nor knowledge was important: the world of explanations and reasons is not the world of existence. A circle is not absurd, it is clearly explained by the rotation of a straight segment around one of its extremities. But neither does a circle exist. This root, on the other hand, existed in such a way that I could not explain it.