This short, early treatise undertakes to explain the controversial thesis that all souls—this includes individual human souls and the world-soul—are in some sense just one soul. Plotinus considers a number of objections that can be raised against the thesis. For instance, if the thesis is true, it may seem to follow that that everything that holds for one soul also holds for every other soul. Plotinus seeks to explain that such objections are off the mark, which also shows that the thesis about the unity of soul is not a claim about strict identity of all souls. He then (chapter 5) considers and rejects two materialistic hypotheses about the relationship between individual souls and the world-soul. In the final chapter he presents his own solution: the individual souls and the world-soul are all the soul hypostasis. He likens the relationship between the hypostatic soul and the individual souls with that between a science and the theorems of the science: each theorem potentially contains the whole science and can in a sense be said to be it.

Eyjólfur K. Emilsson is Professor Emeritus of Philosophy at the University of Oslo, Norway. He received his Ph.D. from Princeton University in 1984 with a dissertation on Plotinus. He is the author of Plotinus on Sense-Perception (Cambridge University Press, 1988), Plotinus on Intellect (Oxford University Press, 2007), [with Steven K. Strange] Plotinus: Ennead VI.4 and VI.5: On the Presence of Being, One and the Same, Everywhere as a Whole (Parmenides Publishing, 2015) and Plotinus (The Routledge Philosophers series, 2017, 2nd edition, 2025). He is the translator of many Platonic dialogues and of other ancient philosophical works, mainly into Icelandic, but also into Norwegian and English.