Certum est quia impossibile est

Certum est quia impossibile est is Latin for "[This] is certain because [it] is impossible." Its origin is the early Christian philosopher Tertullian (160-225 CE), where it appears in De Carne Christi, Chapter 5: 
 * Crucifixus est dei filius; non pudet, quia pudendum est.
 * Et mortuus est dei filius; credibile prorsus est, quia ineptum est.
 * Et sepultus resurrexit; certum est, quia impossibile.


 * The Son of God was crucified: I am not ashamed — because it is shameful.
 * The Son of God died: it is immediately credible — because it is silly.
 * He was buried, and rose again: it is certain — because it is impossible.
 * — Translation by Ernest Evans, 1956.

Often misquoted as credo quia absurdum (I believe because it is absurd), its meaning has long been seen as a rejection of reality rationalism in favor of faith, a view dating back to at least Sir Thomas Browne (1605-1682 CE). However, De Paenitentia Chapter 1, verse 2 shows Tertullian believed nothing of the kind.

Instead, as John Moffatt observed in 1916, certum est quia impossibile est is an argument in the tradition of reason (as it existed at the time of Tertullian) in the line of one of Aristotle's forms as detailed in Rhetoric 2.23.21:


 * 21. Another line of argument refers to things which are supposed to happen and yet seem incredible. We may argue that people could not have believed them, if they had not been true or nearly true: even that they are the more likely to be true because they are incredible. For the things which men believe are either facts or probabilities: if, therefore, a thing that is believed is improbable and even incredible, it must be true, since it is certainly not believed because it is at all probable or credible.
 * — Translated by W. Rhys Roberts

Tertullian would thus seem to be using the reasoning of his time to argue in favor of the resurrection of Jesus Christ by arguing its merit based on the incredibility of it; however, given the greater aims of De Carne Christi, exactly what Tertullian was trying to accomplish in these three lines is not certain.

Beyond that, the incredibility argument as detailed by Aristotle, used by Tertullian, and immortalized in this phrase, has since been abandoned as a reasonable argument. People's beliefs, in and of themselves, are simply not a good indicator of the truth of any matter. History backs this analysis as a great many improbable things people have been willing to believe have, in time, proven not to be true. Further, advances in human psychology and the understanding of how beliefs are formed have undercut Aristotle's basic reasoning in this case.

The argument may be paraphrased as, "Christianity is too weird to have been invented." Any who find that line of thinking convincing are advised to consider what it says about other faiths, such as Hinduism or Scientology.