This early treatise (only second in Porphyry’s chronological list) is a survey, in a rather scholastic mode, of a sequence of doctrines on the nature of the soul adopted by other schools – Epicureans, Stoics, Pythagoreans who hold that the soul is merely a harmony of the body, and Aristotelians – in ascending order of acceptability (§§1–85), leading up to an exposition of the true Platonic doctrine that the soul is not a body, but an immaterial, eternal substance (§§9–15).
A peculiarity of this treatise is that a number of chapters (now numbered 81–85), containing much of the critique of other schools, were omitted from the existing manuscripts of the Enneads, and appear only in extracts from the tractate quoted by the Church historian Eusebius. Since these were not known to Marsilio Ficino, the first editor of the Enneads, who is responsible for the division of the treatises into chapters, they have had to be added to the original numbering.
§1. An introductory chapter, spelling out the nature of the human being, and the respective roles of body and soul.
§§2–4. Plotinus proceeds to a refutation, first, of the materialist psychology of the Stoics (and incidentally, at the beginning of §3, of the Epicureans), refuting the notion that the soul is any kind of material entity, atomic or otherwise, nor yet a pneumatic entity (in the Stoic sense), nor yet a ‘mode’ of body (§4).
§5. Body cannot be a principle either of existence or of movement.
§§6–7. If the soul were a body, it would not be possessed of sense-perception, at least in any conscious or coherent way, nor could it properly analyse the source of pains or other sensations; this against the Stoics.
§8. If the soul were a body, it would not be capable of thought.
§81. Soul is not a quantity.
§82. If soul were material it would not penetrate bodies entirely, as in fact it does; this serves as a rejection of the Stoic doctrine of total mixture.
§83. Soul and intellect are naturally prior to nature and to body.
§84. Refutation of (a misunderstanding of) Pythagorean doctrine that the soul is a ἁρμονία or ‘attunement’ of body, and nothing more than that (the suggestion of Simmias in Plato’s Phaedo).
§85. Refutation of the Aristotelian doctrine (in On the Soul 2.1) of the soul as entelekheia, or ‘realized actuality’ of the body.
§9. The soul as a principle of life, being life of itself.
§10. Soul is of a divine nature; when it discovers its own true nature, that endows it with happiness.
§§11–12. The soul is of its nature immortal and indestructible; having no parts, it is not liable to alteration, or, therefore, dissolution.
§13. The purely intellective part of soul does not descend into body; only that which acquires desire comes into relation to body, without actually being in the body; it produces, embellishes, and directs all things in this world.
§14. Even the souls of non-human living things subsist separately from their bodies, though an element of their souls derives from nature.
§15. Concluding theological postscript. Evidence adduced from divine pronouncements, prophetic shrines, and suchlike.