The winds and elements of the outer universe, the macrocosm, become in Hildegard’s later schemes intimately related to structures and events within the body of man himself, the microcosm, the being around whom the universe centres. The terms _macrocosm_ and _microcosm_ are not employed by her, but in her last great work, the _Liber Divinorum Operum_, she succeeds in most eloquent and able fashion in synthesizing into one great whole, centred around this doctrine, her theological beliefs and her physiological knowledge, together with her conceptions of the working of the human mind and of the structure of the universe. The work is thus an epitome of the science of the time viewed through the distorting medium of this theory. In studying it the modern reader is necessarily hampered by the bizarre and visionary form into which the whole subject is cast. Nevertheless the scheme, though complex and difficult, is neither incoherent nor insane, as at first sight it may seem. On the contrary, it is a highly systematic and skilful presentment of a cosmic theory which for centuries dominated scientific thought. As an explanation of the complexity of existence which thinkers of all ages have sought to bring within the range of some simple formula, this theory of the essential similarity of macrocosm and microcosm held in the Middle Ages, during the Renaissance, and even into quite modern times, a position comparable to that of the theory of evolution in our own age. If at times it passed into folly and fantasy, it should be remembered that it also fulfilled a high purpose. It gave a meaning to the facts of nature and a formula to the naturalist, it unified philosophic systems, it exercised the ingenuity of theologians, and gave a convenient framework to prophecy, while it seemed to illumine history and to provide a key and meaning to life itself. Even now it is not perhaps wholly devoid of message, but as a phenomenon in the history of human thought, a theory which appealed to such diverse scientific writers as Seneca, Albertus Magnus, Paracelsus, Gilbert, Harvey, Boyle, and Leibnitz, is surely worthy of attention. In essaying to interpret the views of our authoress on this difficult subject, we rely mainly on the text of the _Liber Divinorum Operum_, supplemented by the beautiful illuminations of that work which adorn the Lucca MS. The book opens with a truly remarkable vision (Plate VI):