How do, according to Aristotle, peirastic arguments, which are employed by nonscientists to put professed scientists to the test, work, and how do they differ from genuine scientific arguments? A peirastic argument succeeds in unmasking a would-be scientist if it establishes an inconsistency among the answers given. These answers may only comprise: propositions which are proper to the field and which everybody can know; propositions which only scientists may know; “common” propositions that everybody, including various sciences, uses in all kind of arguments. On the other hand, a peirastic argument fails to do its job (and can be abused to make a scientist look stupid) if it either features or presupposes in addition propositions which would justify a fallacy (false common propositions), or if it purports to be scientific (even if the argument may be sound). The latter type of bad peirastic argument crucially depend on common propositions where scientific arguments explain by reference to combinations of the primitive items of the science in question.