His greatest work and the sole speculative system to be produced between the final collapse of the Roman Empire and the 11^th century, is Periphyseon, ‘On the Divisions of Nature,’ written 862-866.

According to the system, Nature is the totality of the things that are and the things that are not. Such is the first division of nature into genera. This division may be made in several ways:

• Things perceivable or intelligible, versus things that transcend sense and intellect (e.g., God, being);
• According to the hierarchy of being, each being in relation to beings above it may be said not to be, while the higher in relation to the lower may be said to be. Alternatively, in a corresponding chain of knowing, the lower ‘is’ for the higher but the higher ‘is not’ for the lower;
• Things ‘with which the fullness of this visible world is made complete’ are, but whatever things are still in potentia in their seed, ‘hidden in their causes’ are not.
• Intellectual things are, but material things subject to becoming are not;
• Human nature exhibits a fifth way of division. Sinful human nature is not, while human nature, ‘restored by the grace of God’s only-begotten Son to the original condition of its substance in which it was created in God’s image,’ is.

The 2 genera of nature may be divided into four species:

• What creates and is not created;
• What is created and creates;
• What is created and does not create;
• What neither creates nor is created.

The differentiae of these divisions are as follows: There are two pairs of species. The third is the opposite of the first, just as the fourth is the opposite of the second. Moreover, the fourth is distinguished by its inability to be. ‘[T]he first [species] is understood in the Cause of all things which have and all which do not have being, the second in the primordial causes, the third in those things known by generation in time and place.’

The first species is God, transcendent and self-existent. However, Eriugena asserts that God may be said to be created in creatures, thus raising a marked tension in his thought between the Augustinian theological legacy and Neoplatonic pantheism. Copleston claims that ‘he is not asserting an evolutionary pantheism, and maintaining that nature, in the ordinary sense, is God-in-His-otherness, for he proceeds to explain that when he says that God is made in creatures, he means that God "appears" or manifests Himself in creatures, that creatures are a theophany’ (Copleston, 117).

Some knowledge of God may be achieved, either in a cataphatic or an apophatic way, a la the Pseudo-Dionysius. Specific contradiction between the ways (e.g., ‘God is wisdom,’ ‘God is not wisdom’) is avoided because affirmation only applies names to God metaphorically, unlike the literal use in negation. Following Pseudo-Dionysius, Eriugena says the apparent contradiction is merely verbal and resolved by affirming that God is, for instance, ‘super-’wisdom. ‘Verbally there is no negation in the predicate ‘super-Wisdom’, but in regard to the mind’s content there is a negation. The via negativa is thus fundamental, and as we do not pretend to define what the ‘super’ is in itself, the ineffability and incomprehensibility of the Godhead is unimpaired.’ (Copleston, 118-119.) The categories do not apply to God—not even substance—so transcendent is God. We may learn from creatures that God is but not what God is.

However, Eriugena takes an interesting tack on this affirmation when it comes to the categories of suffering and making. God does not suffer, he assures, but neither can God be a maker, for two reasons: first, God’s preexistence with respect to the world would place God in time and make creating an accident of God; second, even if creation is eternal and identical with God, motion cannot be attributed to God and so creating cannot be attributed to God. ‘When we hear that God makes all things, we should understand nothing else but that God is in all things, i.e. is the essence of all things. For He alone truly is, and everything which is truly said to be in those things which are, is God alone.’ (I, 72, quoted in Copleston, 120.) Yet he reaffirms creatio ex nihilo. In regard to the relation of reason and authority, Eriugena gives reason priority but in this sense, that interpretation of revealed truths in scripture must be normed by truth discerned by reason; theological tradition is a tradition of more and less reasonable considerations of revealed dogma. Copleston notes that there was no clear separation of philosophical and theological departments of inquiry in Eriugena’s time, as there would be for High Scholasticism.

The second species is the divine ideas or primordial causes existing in the Word of God, ‘created’ in the sense that they are part of the eternal procession of the Word. They are the prototypes of worldly things. There is ambiguity in Eriugena’s claim that they are not a preexisting ‘stuff’ from which God creates this world and in which, therefore, all worldly things participate. Though the primordial causes are multiple, they remain one in the Word, and thus Eriugena denies ontological plurality. There is but one ontological principle in nature, God. Indeed, from the perspective of their status as divine ideas, the causes are divine.

Nonetheless, the third species Eriugena calls ‘participations’ in the primordial causes, as the latter participate immediately in God—a distinctly Neoplatonic emanationist simile.

Creation is a theophany, but what is the nothing from which it was created, if it is nothing other than God? Since God transcends intellect God is duly nothing, so creation may be considered a divine process from nothing into otherness. God ‘comes to be’ in creation. The term of this process is that God be all in all, a motif Eriugena picks up from I Cor. 15:28.

Eriugena’s account of the God-world relation is ultimately ambiguous. The world is created by God and so is other than God. The world is also eternal, for it is not outside God: its logos and primordial causes are in God, it participates in God, and God participates in it, and it will ultimately be assumed back into God when God will be all in all.

Thus, the fourth species is God, understood as that final cause toward which creation tends. All the divisions of nature, then, indicate that the only true reality is God.

Human nature is distinguished by the presence of a rational soul in the person. While it may be correct to consider human nature as the rational species of the genus animal, it is more important that the human soul is made in God’s image. That the image exists can be known by its effect (human nature), but what it is, as being a cause in God, transcends our knowledge. Thus humans participate in God but cannot grasp equality with God. By its standing in the hierarchy, human nature is a microcosm summing up creation.

The uniquely incarnate Word leads fallen humankind back to God. The Word assumes human nature in order to redeem human nature, in which all persons have solidarity. The relation of Incarnation to the (rather Neoplatonic) stages of return of the soul to God, let alone the overall cosmic return, is unclear. Deification is meant to be understood not as a reabsorption into divine substance, but as a kind of return to human nature as it is in its causes. Yet, the resurrection of the body is, by turns, caused by grace or by a natural telos. Eriugena works out damnation as a kind of privation: all human persons will rise with spiritualized bodies and will possess all natural goods, but only the elect will be deified.

Eriugena, The Division of Nature (extracts), in Baird and Kaufmann, Philosophic Classics, 2^nd Ed., From Plato to Nietzsche (Upper Saddle River, N.J.: Prentice-Hall, 1997), 349-352.