This is an early treatise (no. 8 in Porphyry’s list), and concerns an issue that Plotinus inherits from his Platonist predecessors (raised previously also in 4.2 (4), and which continues to concern him until he effectively settles it in the first section of 4.3 (27) (§§1–8), the relation between the individual soul and the soul of the cosmos, or the hypostasis Soul (from which he has not yet clearly distinguished the soul of the cosmos). Plotinus is concerned to argue that all souls are in some sense, or to some extent, one, while maintaining that their diversity must also be recognized.
§1. The thesis that all souls are one is set out, and certain difficulties are raised: what is this single soul, and how do we explain the differences between souls?
§2. These difficulties answered: even within one body, not every part experiences what affects other parts; and diversities may be tolerated within the ambit of a single soul.
§3. The phenomenon of cosmic sympathy argues for the unity of souls, while the diversity of levels of soul does not militate against it.
§4. If the one soul were a body, then the souls deriving from it would be separate, but, as it is incorporeal, the individual souls need not be separate from it.
§5. The question is raised how one single substance can be present in a multiplicity of souls. The answer is that this is not a problem for an immaterial entity; a good analogy that is proposed is that of a science and its individual propositions, each of which assumes the whole science.