From the transmigrating soul of Pythagoras to Aristotle's form of the body, the immortal spirit of the theologians, and the modern suspicion that the 'ghost in the machine' is a mistake.
Is there a soul — and if so, what is it: an immortal substance, the form of a living body, or a picture we should abandon?
For most of human history 'soul' named whatever makes a body alive, aware, and itself. The Greeks split over whether it is an immortal traveller trapped in flesh or simply the organizing principle of a living thing; the monotheistic traditions made it the seat of personal immortality and moral responsibility before God. Early modern philosophy hardened the soul into an immaterial thinking substance, sharply divided from the body — and thereby created the problem of how the two could ever interact. Meanwhile the Buddhist tradition denied there is any permanent self behind experience at all, and twentieth-century analysis mocked the idea of a 'ghost in the machine'. Set side by side, these views disagree not only about whether the soul survives death but about whether 'soul' points to a thing, a function, or an illusion.
Pre-Socratic / Pythagorean
The soul is an immortal, transmigrating principle that passes through successive bodies, human and animal, in a cycle of rebirth. Purification through mathematics, music, and an ascetic way of life frees the soul from the body's contamination and its 'wheel of birth'. On this view the body is a temporary vessel and even a kind of tomb for a divine wanderer.
Doctrine of metempsychosis, reported by Porphyry and Iamblichus; influence on Plato.
Ancient Greek (Academy)
The soul is immortal and pre-exists the body, into which it descends and from which it is separated at death. It is tripartite — reason, spirit, and appetite — and justice in a person is the rule of reason over the lower parts. In the Phaedo Plato argues the soul's kinship with the eternal Forms shows it cannot perish with the body.
Phaedo; Phaedrus; Republic, Book IV (the tripartite soul).
Ancient Greek (Peripatetic)
The soul is not a separate thing lodged in the body but its form — 'the first actuality of a natural body having life potentially within it'. Soul and body are related as shape to matter, so in general the soul cannot survive the body's dissolution any more than a stamp survives the wax. Plants, animals, and humans differ by nutritive, perceptive, and rational capacities, and only the active intellect raises a puzzle about separability.
On the Soul (De Anima), Books II–III.
Neoplatonism
Soul is the third hypostasis, emanating from Intellect, which itself flows from the One; it mediates between the intelligible and the sensible worlds. The individual soul is not fully descended into the body — a 'higher part' remains contemplating the intelligible — and salvation is its ascent back toward its source. Matter is the last, faintest trace of the outflow, and the soul's fall into it is the root of forgetfulness.
Enneads (esp. IV, on the soul).
Christian (Patristic)
The soul is an immaterial, immortal substance, made in God's image and superior to the body it governs. Turned inward, the soul discovers in memory, understanding, and will a created trace of the Trinity, and in its restless self-knowledge it finds God 'more inward than my innermost'. Its immortality is not natural self-sufficiency but a gift ordered to eternal union with God.
Confessions; On the Trinity; On the Immortality of the Soul.
Islamic Peripatetic (Falsafa)
Adapting Aristotle, Avicenna argues the rational soul is an immaterial substance, not merely the body's form, and therefore survives it. His 'Flying Man' thought experiment — a person created suspended in the void, without sensation — shows the soul affirms its own existence prior to any awareness of the body. The soul uses the body as an instrument but is not identical with it.
The Book of Healing (Kitāb al-Shifāʾ), 'On the Soul'; the 'Flying Man' argument.
Scholasticism
The soul is the substantial form of the body, as Aristotle held, yet because the intellect's operation transcends matter, the rational soul is subsistent and survives death. But the separated soul is incomplete — it is not the whole person — which is why Aquinas holds the resurrection of the body to be necessary for full human beatitude. Soul and body compose one substance, not two things loosely joined.
Summa Theologiae, I, qq. 75–89.
Rationalism / early modern
The soul is a thinking, non-extended substance (res cogitans), really distinct from the extended body (res extensa); its whole essence is thought. I can doubt that I have a body but not that I think, so mind and body are metaphysically separable, and the soul is naturally immortal. The two nonetheless interact intimately, Descartes suggested, through the brain — a union he never satisfactorily explained.
Meditations on First Philosophy (1641); The Passions of the Soul (1649).
Early Buddhism
There is no permanent, unchanging self or soul (ātman) underlying experience: the person is a stream of five ever-changing aggregates — form, feeling, perception, mental formations, and consciousness. What we call 'I' is a convenient designation for this flux, and clinging to a substantial soul is a source of suffering. Rebirth occurs, but as causal continuity, not the transfer of a self.
Anattalakkhaṇa Sutta; the doctrine of the five aggregates (khandhas).
Empiricism
Introspection never reveals a simple, continuing self or soul-substance, only a bundle of perceptions in constant flux. We are 'nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity'; the idea of a persisting soul is a fiction the imagination fabricates from resemblance and causation among these perceptions. There is no impression of the soul, so there is no legitimate idea of it as a substance.
A Treatise of Human Nature (1739–40), Book I, Part IV, 'Of Personal Identity'.
German idealism / critical philosophy
The rational-psychology proofs of the soul as a simple, immortal substance are fallacies — the 'paralogisms of pure reason' — because they mistake the formal unity of the 'I think' for knowledge of a thing. We can never encounter the soul as an object of experience. Yet the immortality of the soul remains a rational postulate of practical reason, required to make sense of the moral striving toward the highest good.
Critique of Pure Reason (1781), 'Paralogisms'; Critique of Practical Reason.
Ordinary-language / analytic philosophy
The Cartesian soul is 'the ghost in the machine', a category mistake that treats mental life as an inner immaterial theatre parallel to the bodily one. Mind-talk does not name a hidden substance but refers to dispositions and capacities exhibited in intelligent behaviour. To understand mind we should look at how people act, not posit a spectral inner self pulling the body's levers.
The Concept of Mind (1949).