How to Win Every Argument: The Use and Abuse of Logic (2006)

Cireulus in probando 27

Cireulus in probando

Cireulus in probando is a specialized and very attractive form of

the petitio principii. It consists of using as evidence a fact which is

authenticated by the very conclusion it supports. It is thus

arguing in a circle.

7 didn't do it, sir. Smith minor will vouch for my honesty.'

'Why should I trust Smith minor?'

'Oh, I can guarantee his honesty, sir. '

(Any teacher who falls for that one deserves to be suspended by his

thumbs from two hypotheticals.)

The cireulus is fallacious for the same reason as is its larger

cousin, petitio. It fails to relate the unknown or unaccepted to the

known or accepted. All it gives us is two unknowns so busy

chasing each other's tails that neither has time to attach itself to

reality.

We know about God from the Bible; and we know we can trust the Bible

because it is the inspired word of Cod.

(A circle in a spiral, a wheel within a wheel.)

As with the petitio, its close relative, the cireulus is often found
building a cosy little nest in religious or political arguments. If
there really were convincing proofs of particular religions or
ideologies, it would be much more difficult for intelligent people
to disagree about them. In place of cast-iron demonstrations,
petitio and cireulus are often called upon to serve.
The same could even be said of science. How do we know that
our so-called scientific knowledge is no more than one giant
cireulus? When we perform scientific experiments, we are
assuming that the rest of our knowledge is good. All we are really