 Another Augustinian aspect of Mersenne’s defense is the idea that doubt itself can be turned into a source of knowledge. For example, although there are differences in apprehension between various men, or between men and beasts, we do have the knowledge of such differences and this is something that cannot be doubted: “at least one knows that the objects of the senses appear differently according to the diverse dispositions of the organs” (VS: 193). Awareness of this sort of facts is what incites men to seek the laws of appearances—for example, the optical and geometrical rules that govern variations in perspective. In other terms, it prompts them to engage into physico-mathematical enquiries.