What is Circulus In Probando?

Circulus 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.


‘I 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 circulus 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 God.

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


As with the petitio, its close relative, the circulus 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 circulus 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 circulus1 When we perform scientific experiments, we are assuming that the rest of our knowledge is good. All we are really testing is whether the new theory under examination is consistent with the rest of our theories. At no point can we test any of them against some known objective truth. After all, even the theories about what our senses tell us are in the same predicament. It all comes down to saying that science gives us a consistent and useful look at the universe through the ring of a giant circulus.

You will find it difficult, however, to use the prestige of science in support of your own use of the circulus. He is too easily spotted for effective application in argument, being rather less wily than his big cousin, petitio.


‘I have the diamond, so I shall be leader.’

‘Why should you get to keep the diamond?’

‘Because I’m the leader, stupid.’


The more likely your conclusion is to be acceptable for other reasons, the more likely are you to get away with a circulus in support of it. When people are already half-disposed to believe something, they do not examine the supporting arguments as closely. That said, circulus should be reserved for verbal arguments where memories are short.


‘I’m asking you to do this because I respect you.’

‘How do I know that you respect me?’

‘Would I ask you to do it otherwise?’

(If you want to do it, you’ll believe it.)


The intelligent reader might suppose that fallacies such as circulus are too obvious to be more than debating tricks. Surely they could never seriously distort decisions of state by slipping through the massed ranks of civil servants, government committees and the cabinet? Not so. A major policy of Britain’s government in the 1960s, adopted after the most serious public debate, was based upon a relatively obvious circulus in probando. This was the National Plan, an exercise in (then fashionable) national economic planning. Firms were asked to assume a national growth-rate of 3.8 per cent, and to estimate on that basis what their own plans for expansion would be. These various estimates were added up by the government, which concluded that the combined plans of British industry suggested a growth rate of 3.8 percent!

The National Plan was valueless then and subsequently, except to connoisseurs of logical absurdity lucky enough to snap up remaindered copies of it in secondhand bookshops.

The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!