Galen claims that in his youth—partly in response to the confusion caused by mutually contradictory philosophical sects, all claiming certainty on unprovable issues—he nearly succumbed to the lures of Skepticism (Lib. Prop. 14/11, 18). The above possibility of certainty, as provided by mathematics and geometry, is what saved him. But an understanding of the limits of human knowledge is as important as the project of acquiring it in those areas where that is possible. Galen is scathing about those who commit themselves rashly on insufficient evidence, and about philosophers who debate propositions on which demonstrative knowledge is impossible (such as the existence and nature of the void outside the cosmos) (Aff. Pecc. Dig. II.6–7, 308–13). Here, one criterion to which he appeals in refusing to make pronouncements on certain questions is their lack of relevance to “medicine or ethical/practical philosophy” (e.g., Prop. Plac. 15, 120; PHP IX.9, 600).