1. There is a distinction between being, which is "that which is known (or accessible to sensation)" and non-being, which is not so accessible.
The first distinction between being and non being reflects Eriugena's neo Platonic roots. Being is that which is known by the human mind and non being is all else. This is the highest distinction, and places God in the realm of non being. That which eludes sense and reflection does so by the "excellence of its nature" (I.3). But not only God fits into this category; so too does ousia, or substance. On this level it is the mind that gives things their being. That which transcends the mind, therefore, cannot be subject to it, and cannot have being. Being and non being, therefore, are predicated on knowledge, as is the case for Parmenides and Pseudo Dionysius. That which cannot be known, also cannot be. It should be noted that, despite the fact that Eriugena follows Plotinus here, he does not emphasize non being as unity, but rather non being as pre eminence. In fact, he does not mention unity at all in his discussion of the first method, and it seems he could not if he wanted to, since both God and ousia qualify as non being in this sense.
2. If being is sameness, non-being amounts to difference. This is a kind of perspectival sense of non-being, which depends on the structure of Porphyry's tree.
In his second method, non being is perspectival. That which is depends on what part of the Porphyrian tree one considers, and the levels above do not contain the levels below, but are the negation of the levels below, as well as themselves negations of the levels above. In this way, the species negates the genus. This deviates from Plotinus, since for Plotinus the species is always contained in the genus. The higher gives rise to and contains the lower. For Eriugena the lower is the negation of the higher, and the higher is the negation of the lower. Dermot Moran points out that this gives rise to a kind of perspectivism. It is, however, only a partial perspectivism; God is not, after all, part of this distinction. This distinction begins "from the most exalted intellectual power stationed closest to God." The non being of God is not established simply on the basis of the negation from the level below God. God's non being is more profound than that.
3. That which exists can be said to be, that which does not yet exists (or exists in potential) can be said to not be. Future things are-not.
He includes potentiality as one type of non being. In this one, he points out that that which does not yet exist can be said to not be. Eriugena distinguishes this mode from the first (in which there is a distinction between being as that which is known, and non being, which is all else) by pointing out that in the first mode, it is a question of knowledge of things "together in their causes and effects", or intellection of things in totality, whereas the third mode has to do just with knowledge of things revealed in their effects. Something hidden in its cause, in the third mode, would be said to not be, while in the first, it would be said to be. With this, he provides for the possibility of generation and corruption.
4. That which is permanent has being, and that which changes or is subject to corruption does not.
Eriugena's fourth distinction owes the most to Parmenides and Plato, and is the opposite of the third. In this mode, that which is permanent is said to have being, whereas that which changes through generation or corruption does not have being. Only those things grasped by the intellect have being, and since only permanent things can be grasped by the intellect, only permanent things have being.
5. Sin is a kind of non-being. For the mediaevals, Truth = Goodness = Being = Beauty. Thus, sin separates us from being.
John Scotus Eriugena's fifth distinction follows Augustine more closely than his others in referring to the human nature tainted by sin as not having being. Here there is a sense of "rectitude" which we also see in Augustine -- whatever falls out of its proper place in the state of grace has fallen out of being as well.