While the scheme of the universe put forward by ancient and medieval science is, as we have seen, on the whole adopted even in Hildegard’s most visionary writings, it is equally true that the religious interest is by no means absent from her two works of medicine and natural history. In the first place, the devil is a force in nature which she often mentions. Her opening the _Causae et curae_ with a discussion of creation--of course a usual starting-point with the medieval scientist--soon leads her to speak of the fall of Lucifer. She has a rather good theory that Lucifer in his perverse will strove to raise himself to Nothing, and that since what he wished to do was Nothing, he fell into nothingness and could not stand because he could find no foundation under him.[371] But after the devil was unable to create anything out of nothing and fell from heaven, God created the firmament and sun, moon, and stars to show how great He was and to make the devil realize what glory he had lost.[372] Other creatures who willingly join themselves to the devil lose their own characteristics and become nothing.[373] Lucifer himself is not permitted to move from Tartarus or he would upset the elements and celestial bodies, but a throng of demons of varying individual strength plot with him against the universe.[374] But in other passages Hildegard seems to admit freely the influence, if not the complete presence, of the devil in nature. And he has the power of deceiving by assumed appearances, as Adam was seduced by the serpent. [Sidenote: Natural substances and evil spirits.]