Poverty of the stimulus (POS) arguments have instigated considerable debate in the recent linguistics literature. This article uses the comparative method to challenge the logic of POS arguments. Rather than question the premises of POS arguments, the article demonstrates how POS arguments for individual languages lead to a "reductio ad absurdum" as POS arguments from genetically related languages are compared. Comparison leads to different contradictions for poverty of the negative stimulus (PONS) and poverty of the positive stimulus (POPS) arguments. Comparing PONS arguments leads to the conclusion that Universal Grammar contains language-specific versions of linguistic rules. Comparing POPS arguments leads to the conclusion that Universal Grammar may supply knowledge that is ungrammatical in the target language. The "reductio" shows that universal principles of grammar cannot be established on the basis of POS arguments from a single language. (Contains 1 footnote.)