 The Periphyseon, deeply influenced by Eriugena’s engagement with Greek Christian authors, is a work of astonishing scope, a veritable Christian Neoplatonic summa. At the beginning of Book Four, Eriugena labels his enterprise a physiologia, a “study of nature”, and indeed one manuscript of the Periphyseon in the British Library in London calls the whole dialogue “Liber Phisiologiae Iohannis Scottigenae”. The term “physiologia” is apt in that the term “nature” for Eriugena spans the whole cosmological domain, including not just created nature but also the Divine Creator, and treats of the essentially dialectical relation between Creator and created, where God expresses Himself in creation and creation culminates in return to the divine. Nature is to be understood as what is real in the widest sense, the totality of all things that are and are not. Nature includes both God and creation and has four divisions: nature which creates and is not created (God), nature which creates and is created (the Primordial Causes), nature which is created and does not create (the Created Temporal Effects), and nature which is neither created nor creates (Non-Being). The original intention (expressed at III.619d–620b) was to devote one book to each of the four divisions; thus Book One deals with the divine nature and the procession or exitus of all things from God, Book Two treats of the Primordial Causes, and Book Three of their Created Effects, including the nature of ex nihilo creation and the stages of the creation of the world. The topic of creation requires Eriugena to address issues related to the Biblical account of creation, and thus, in Book Three, he embarks on his own version of a Hexaemeron. The momentous event of the emergence of human nature on the Sixth Day of creation requires extended treatment, and Eriugena is forced to devote a fourth book to this topic, thus relegating the return of all things to God to a fifth book. Thus, Eriugena was forced to depart from his original plan of four books and add a fifth. This change of plan is particularly important in that it helps to identify different stages of composition of the text.