When testing logical formulae for validity by applying Venn's diagrams we are using a kind of picture writing i. e. a writing in which certain facts are represented by drawings rather than sentences. This diagrammatic decision procedure may however be translated into ordinary language. Translating for instance the respective procedure of testing the mood Darii we obtain the following reasoning.

The paper gives a tentative reconstruction of the classical theory of so called fallacious arguments. Its title refers to the following observations.One of the fallacies listed in traditional logic ispetitio principii. It seem natural to add to the list another, similar fallacy. An argumentationArg + considered as a part of a theoretical contextC commits this fallacy relatively toC, if it contains an inference such that the principle of this inference has not been proved inC. By principle of a given inference (...) the conditional is meant whose antecedent and consequent are the conjuction of all the premisses and the conclusion of the inference respectively.If the principle of a given inference has been proved in a given contextC, the inference is valid relatively toC and the premisses are implying the conclusion relatively toC. Both these concepts, of valid inference and of implication do involve the concept of an effectively performed proof; hence they are pragmatic concepts. (shrink)