"Aristotle's distinction between fallacies in dictione and fallacies extra dictionem is not the same as Richard Whately's division into logical, and non-logical or material. By "logical" fallacies Whately meant those in which "the conclusion does not follow" from the premisses; by "material," those in which the "conclusion does follow" from the premisses. In the former class, the defect of proof lies either in a manifest violation of some of the formal laws of the syllogism--quaternio terminorum, undistributed middle, illicit major, illicit minor, negative premisses, etc., defects which remain even when symbols are substituted for the terms and concepts, and which Aristotle would not regard as sophisms owing to the transparency of the mistake;--or the defect lies in a similar violation masked in ambiguous language. The transparent defects Whately called purely logical, the cloaked defects semi-logical fallacies. The latter he regarded as all alike reducible to ambiguous middle term, including in this class all Aristotle's sophisms except the ignoratio elenchi, the petitio principii, and the non causa pro causa. These three he included in his "material" falacies, by which he understood mistakes due to assuming false or unproven premisses, or premisses which prove the wrong conclusion."