Soul, mind and body - Classical souls and Cartesian dualism

Soul and conscious experience

The mind-body problem asks how conscious experience relates to the physical organism. Consciousness means awareness and experience, such as feeling pain or seeing red; it is different from conscience, which concerns moral judgement. Metaphysics asks what kinds of reality exist and how they are related. Describing a brain process and explaining what an experience feels like may therefore answer different questions.

The word soul has several uses. It can mean an immaterial self capable of surviving death, a principle that makes an organism alive, or a metaphor for someone's deepest character. These meanings must be distinguished before deciding whether a soul exists. Aristotle's soul is not simply Descartes' thinking mind with a Greek name.

Substance dualism holds that mind and body are fundamentally different kinds of substance: a mental or spiritual subject and a material body. Substance here means something that bears properties, not a chemical substance. This differs from saying that a single physical person has both a weight and a thought; different descriptions do not automatically establish two independently existing things.

An ordinary case makes the issue concrete. A cut involves tissue damage and a felt pain. A dualist thinks the pain is experienced by an immaterial subject related to the body. A materialist thinks physical processes can account fully for the experience. Both can accept that bodily events and experiences are closely connected. Their disagreement is about what that connection ultimately involves.

Plato's separable soul

Plato treats the soul as the essential immaterial part of a person, temporarily united with a body. In the Phaedo, the soul is associated with reasoning and grasping unchanging realities, whereas the body is changeable and can distract through appetite and sensory confusion. Death is understood as separation of soul and body. These are paraphrases of Plato's position.

His recollection argument links the soul to knowledge. We recognise imperfect examples of equality as falling short of Equality itself. Plato reasons that knowledge of the perfect standard precedes these experiences and so indicates the soul's existence before bodily life. The argument's strength depends on whether such recognition really requires prenatal knowledge, rather than concepts formed or developed through experience.

The affinity argument compares the soul with invisible, intelligible and less changeable reality, and the body with visible, divisible and perishable things. Affinity supports the proposal that the soul is suited to survival. A likeness is not by itself a demonstration that the soul cannot perish: the crucial connection between immateriality and immortality must be defended.

In the Republic Plato also distinguishes rational, spirited and appetitive aspects of the soul. Reason should order the other motivations, allowing justice within the person. This moral psychology is not simply a claim that humans have three separate souls. It helps explain why someone can desire an action and judge that it should be resisted.

The value of the account is that a person is more than shifting bodily desires and can orient life towards truth and goodness. Its difficulty is evidence: a proposed separable self must explain its relationship with the embodied person we observe.

Aristotle's soul as living form

Aristotle's soul is the form of a living body: its organisation and capacities as a living thing. In De Anima II.1 he describes it as the actuality of an organised body potentially alive. A soul is therefore not an extra occupant inside an otherwise complete organism. It makes the body the living organism it is.

Plants have nutritive capacities such as growth; animals additionally perceive and move; humans additionally reason. Soul language here covers life more broadly than human consciousness. If an axe were a natural living thing, its defining activity would help explain its soul; Aristotle's own comparison with an eye makes sight the relevant living capacity. A dead eye can retain a shape while losing the activity that made it a living organ.

On this account, the living body and soul are conceptually distinguishable but do not normally exist separately. Asking where the soul is, as though it were an organ next to the heart, misunderstands form. This fits the observation that our capacities are embodied and changes in bodily functioning alter them.

There is an important textual qualification. Aristotle's treatment of intellect, particularly De Anima III.5, raises disputed questions about separability. It is misleading to turn that difficult passage into a simple doctrine that every personal soul survives death. The central account studied here is soul as bodily form, with no detachable personal soul established by that account.

Compared with Plato, Aristotle explains living activities without a separate spiritual self. A critic may ask whether organisation alone accounts for subjective feeling. The answer will depend on whether a description of capacities explains experience itself or only what an organism can do.

Descartes' distinct substances

Descartes distinguishes res cogitans, thinking substance, from res extensa, extended substance. Thought includes doubting, willing and experiencing; extension means occupying spatial dimensions. A body has size and shape and is divisible; the thinking self is understood as a unified subject rather than a spatial object.

Methodical doubt motivates the distinction. Descartes can doubt what he sees and whether a body exists, but the occurrence of doubting demonstrates that a thinking subject exists. This establishes a certainty about thinking; by itself it does not yet demonstrate two substances. Knowing one thing more securely than another is not automatically knowing they are different things.

In Principles of Philosophy I.60–65, his fuller argument appeals to a clear and distinct conception of mind without body and body without mind. He holds that God can create them separately, making them really distinct. The theological guarantee is part of his reasoning; an account that omits it makes separability look like a simple inference from ignorance. Descartes also contrasts the divisibility of body with the unity of mind.

One can reconstruct the reasoning: clearly and distinctly conceivable independent substances are separable; mind can be conceived apart from extension; body can be conceived apart from thought; therefore mind and body are distinct. The difficult premise is the connection between conceivability and actual possibility. A critic may suggest that our limited concepts conceal an identity we have not discovered.

Interaction and explanatory costs

Cartesian dualism must explain how unlike substances interact. An intention appears to cause a hand to move; bodily injury appears to cause pain. If the mind is non-spatial, how does it affect spatial matter? Descartes associated interaction with the pineal gland, but locating a bodily site does not by itself explain the causal connection between the kinds of substance.

The objection is not that dualists deny every brain-mind correlation. A dualist can say the body supplies sensory information or enables a mind to express itself. The problem is whether this reply supplies an intelligible mechanism or merely redescribes the dependence. A materialist offers one physical causal account; a dualist argues that such an account risks leaving out the experiencing subject.

The three accounts therefore solve different problems at different costs. Plato connects rational identity to unchanging truth but must justify separability and survival. Aristotle explains an embodied living unity but faces questions about subjective experience. Descartes makes the thinking subject explicit but inherits the interaction problem and the challenge to conceivability.

There is also a different kind of objection: perhaps treating mind as a second thing alongside body is a category error, putting a concept into the wrong logical category. The next lesson develops Ryle's argument and the materialist alternative. It will distinguish rejecting a spiritual substance from denying that people really think or feel.