In this treatise, Plotinus considers the everlastingness of the universe, the heavens, and the heavenly bodies. In earlier treatises he had already established that the everlastingness of the universe follows from the metaphysics of procession and reversion, but now he needs to show how this is compatible with the universe and the heavens having bodies, given that all body appears to be in flux. Plotinus is convinced that external material flux – that is, matter flowing out of a body – must ultimately undermine the diachronic identity of a composite living thing, and so he sets out to show that there is no such external material flux in the case of the universe, the heavens, and the heavenly bodies. This, in turn, leads him to consider what the elemental constitution of the heavens and heavenly bodies must be if no external flux is to take place.
§1. Plotinus rules out two purported explanations of universe’s everlastingness as inadequate: the will of god and there being nothing outside of the universe.
§§2–4. The soul is the cause of the everlastingness of the universe and the heavens, but the body must cooperate.
§5. Why the celestial living things (stars and planets) are everlasting, while sublunary living things (e.g. human beings) are not, even though both are only parts of the universe.
§§6–7. The elemental constitution of the heavens and the heavenly bodies.
§8. There is no external flux in the heavens, nor do the heavens require any nourishment.