As was stated above, we can date some of Hildegard’s works with exactness. In her preface to the one entitled _Scivias_[344] she says that in the year 1141, when she was forty-two years and seven months old, a voice from heaven bade her commit her visions to writing. She adds that she scarcely finished the book in ten years, so we infer that she was working at it from 1141 to 1150. This fits exactly with what she tells us in the preface to the _Liber vitae meritorum_, which she was divinely instructed to write in 1158, when she was sixty years old. Moreover, she says that the eight years preceding, that is from 1151 to 1158, had been spent in writing other treatises which also appear to have been revealed in visions and among which were “_subtilitates diversarum naturarum creaturarum_,” the title of another of her works with which we shall be concerned. On the _Liber vitae meritorum_ she spent five years, so it should have been completed by 1163. In that year, the preface to the _Liber divinorum operum_ informs us,--and the sixty-fifth year of her life--a voice instructed her to begin its composition, and seven more years were required to complete it. This leaves undated only one of the five works by her which we shall consider, namely, the _Causae et curae_, or _Liber compositae medicinae_ as it is sometimes called, while the _Subtilitates diversarum naturarum creaturarum_ bears a corresponding alternative title, _Liber simplicis medicinae_. [Sidenote: Question of their genuineness.]