77. The Unity of the Soul: One Form or Multiple Souls

Subscribe in Podcast App | Download Transcript

Lecture Notes

Main Topics #

The Problem: Multiple Souls or One? #

• Platonic position: Diverse souls in one body, each with distinct organs (nourishing power in liver, sense desire in heart, knowing in brain)
• Objection from incorruptibility: The understanding soul is incorruptible; the sensing and feeding souls are corruptible; therefore they cannot be of one substance
• Objection from genus: If the sensitive soul in man is incorruptible but in animals is corruptible, they differ in genus, making “animal” equivocal when predicated of man and brute animals
• Objection from embryology: The embryo is an animal before it is a man, suggesting successive souls at different developmental stages
• Objection from form and matter: Genus comes from matter, difference comes from form; therefore the understanding soul (form) cannot be the same as the sensing soul (more material)

Aristotle’s Refutation of Plato #

• The soul cannot be related to the body as a mover to the moved (as Plato held—like a sailor in a ship)
• If the soul is the substantial form of the body, then multiple souls differing in nature cannot exist in one body
• A thing has both being and unity from the same principle; if there were multiple forms, there would be multiple beings, not one unified being

The Key Principle: Substantial Form #

• Substantial form causes a thing to be simply and without qualification
• Accidental form gives existence in a limited, qualified way (like health or knowledge)
• A thing has the unity and being it has from its substantial form
• Therefore, nothing can be simply one except through one form

The Solution: One Soul with Multiple Powers #

• The rational soul is the single substantial form of the human body
• This one soul has multiple operative powers: vegetative, sensitive, and rational
• These are not separate souls but different operations of one unified substantial form
• The soul vivifies the body (vegetative), gives sensation (sensitive), and enables understanding (rational) all through its one substantial form

The Problem of Predication #

• When we say “man is animal,” this is not accidental predication
• It must be per se predication (essential, not incidental)
• This requires that the same form makes something both animal and man
• If different forms were involved, “animal” would be equivocal when applied to man versus brute animals
• Genus must be univocal across all species within it

The Embryological Development #

• The same rational soul is present from conception, not a succession of different souls
• The soul’s operations depend on the development of bodily organs
• As organs develop, the soul’s operations become manifest progressively
• The embryo is potentially an animal before it actually functions as one; the rational soul is present throughout but operates according to bodily capacity

Key Arguments #

Thomas’s Central Argument Against Multiple Souls #

1. Nothing is simply one except through one form

   • A thing has being and unity from the same principle
   • If man had different forms (vegetative, sensitive, rational), he would not be one thing simply

2. Therefore, there must be one form (the rational soul) that gives all life

   • The sensitive soul in man is not a separate form but a power of the rational soul
   • The vegetative soul is not separate but another power of the same rational soul

3. Verification from the predication “man is animal”

   • This is an essential, not accidental, predication
   • Requires the same form to constitute man as animal and as man
   • If different forms, then “animal” would mean different things in man versus brute

Important Definitions #

Substantial Form (forma substantialis) #

• The principle by which a thing has being simply and absolutely
• Comes to a subject in potency (not yet actual)
• Gives existence without qualification
• The human soul is the substantial form of a living human body

Accidental Form #

• Comes to an already actual subject
• Gives existence in a limited, qualified way
• Examples: health, knowledge, color, shape
• Not essential to the being of the subject

Per Se Predication (praedicatio per se) #

• First mode: Predicate is in the definition of the subject (e.g., “number is divisible”)
• Necessary for scientific demonstration
• Contrasts with accidental (per accidens) predication, which cannot serve as basis for demonstration

Univocal vs. Equivocal Predication #

• Univocal: Same term applied with same meaning to multiple things
• Equivocal: Same term applied with different meanings
• Genus must be univocal across all species within it (e.g., “animal” means the same thing in man and in dog)

Examples & Illustrations #

The Embryological Example #

• Cell division and growth occur first (plant-like operations)
• Then sensation appears (animal operations)
• Finally, recognizably human understanding emerges
• Demonstrates progressive manifestation of one soul’s operations, not replacement of souls

The Worm Example #

• When a worm is cut, each part exhibits sensation and pain
• Sensation is not localized in one part but distributed through the unified organism
• Shows that soul operations are not in distinct locations but emanate from one unified principle
• Demonstrates the sensory soul is not separate from the principle of life

The Liver and Heart in Classical Thought #

• Platonists placed nourishing power in the liver
• Platonists placed sense desire in the heart
• Different organs for different operations
• Thomas rejects this: all operations proceed from one soul with different powers

The Intensity Interference Principle #

• Emotional upset (animal nature) impedes thinking
• Intense intellectual study can impede devotion and prayer
• This interference proves operations come from one unified source
• Separate souls would have separate operations that could not interfere with one another

Notable Quotes #

“Nothing is simply one except through one form, to which a thing has existence. And from the same thing, a thing has that it is a being, and that it is one.” — Thomas Aquinas, on the unity of substantial form

“If therefore, a man from another form had that he is alive, namely from the vegetable soul, and from another form, that he is an animal, namely from the sensible soul, and from another, the man, namely from the rational soul, it would follow that man is not one thing simply.” — Thomas Aquinas, refuting multiple souls

“One operation of the soul, when it becomes very intense, impedes another. Which in no way would happen unless the principle of actions was one in its very nature.” — Thomas Aquinas, on the empirical sign of soul’s unity

Questions Addressed #

Q: How can the corruptible and incorruptible be of one substance? #

A: The sensitive soul in man is incorruptible not by its own nature but because it is a power of the rational soul, which is incorruptible. The rational soul elevates all its powers to incorruptibility.

Q: If sensitive soul is incorruptible in man but corruptible in animals, isn’t “animal” equivocal? #

A: No. The sensitive soul in man is incorruptible precisely because it is a power of the rational soul, not as a separate form. The same generic principle of sensation operates in both, though elevated in man.

Q: How does the embryo become a man if it starts as plant, then animal? #

A: The same rational soul is present from conception. As the body develops its organs, the soul’s operations become manifest progressively. It is not a succession of different souls but the actualization of one soul’s powers according to bodily development.

Q: Why does intense emotion impede thought if they are different powers of one soul? #

A: Because they are operations of the same unified soul. When one operation becomes very intense, it absorbs the soul’s attention and capacity, leaving less for other operations. This empirical fact proves the operations come from one source.

Connections to Broader Framework #

Metaphysical Foundation #

• Based on the principle that being (esse) and unity come from the same source
• Substantial form is the ultimate principle of both being and unity in composite substances
• This applies specifically to human nature as a unified composite of body and soul

Relationship to Predication and Logic #

• The ability to predicate “man is animal” per se requires the unity of form
• Demonstrates how metaphysics grounds logic
• Shows why equivocal predication cannot serve as basis for scientific knowledge