28. Light, Transparency, and the Rejection of Corpuscular Theory

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Lecture Notes

Main Topics #

Aristotle’s Account of Light and the Transparent #

• Light is the actuality of the transparent, not a body or stream of particles
• Light is a form or quality of the transparent, comparable to how health is a form of the body
• The transparent (τὸ διαφανές) is that which is visible through an extraneous color
• Darkness is the privation or lack of light in the transparent
• Light is the presence of fire or lucid body actualizing the transparent

Rejection of Empedocles’ Corpuscular Theory #

Aristotle rejects the view that light consists of subtle bodies spreading through space, giving two main arguments:

1. The Spatial Occupancy Argument: Two bodies cannot occupy the same place simultaneously. If light were a body spreading through the room, and air is also a body in the same place, this is impossible.

2. The Temporal Argument: If light were locomotion (bodies moving through space), it would require time. Yet illumination appears instantaneous across vast distances (from east to west). Therefore, illumination is not a locomotion.

   • Logical form: If A then B; not B; therefore not A (modus tollens)
   • Critical weakness: The second premise—that illumination takes no time—is only probable, not necessary. Modern measurements show light travels at ~186,000 miles per second.

Modern Physics and Historical Development #

• Newton and Huygens debated light’s nature: Newton favored particles; Huygens favored waves
• 19th-century experiments (e.g., diffraction) supported the wave theory for ~200 years
• Einstein (1905) explained the photoelectric effect using light quanta (later called photons), reviving corpuscular theory
• Wave-particle duality: Modern physics shows light behaves as both waves and particles depending on experimental conditions
• Berquist notes that Aristotle’s ancient adversary (Empedocles) was closer to modern science than Aristotle himself

The Role of the Medium in Vision #

• Color must move the transparent in act for vision to occur
• The transparent must be actually transparent (illuminated) for color to be perceived
• Without a transparent medium, no vision is possible
• Criticism of Democritus: He believed that if the space between the eye and object were empty, we would see more clearly (even ants in the heavens). This is impossible because seeing requires the sensitive organ to undergo something; color must move the transparent medium.

Common Experience vs. Private Experience in Philosophy #

• Greek approach: Attempted to go as far as possible using common experience, then acknowledged limits and pursued private experience where necessary (e.g., Aristotle’s dissections and 150 city-state studies)
• Modern approach: Denies that anything can be known from common experience alone, making everything dependent on specialized instruments and private experience
• Berquist’s assessment: Greeks are less culpable because no one could know in advance how far common experience extends; moderns are more irrational in rejecting common experience altogether
• Principle of simplicity: One should not demand more experience than necessary to know something

Logical Tools for Reason: Definition and Demonstration #

• Definition (like a microscope): Magnifies or expands something to see it more distinctly and minutely
• Demonstration/Syllogism (like a telescope): Enables reason to reach distant truths through a chain of premises and conclusions, each step building on the previous one (as in Euclid’s progression to the Pythagorean Theorem)
• Neglect by moderns: Modern philosophers (e.g., John Stuart Mill) deny the validity of definitions and syllogisms, yet paradoxically use them in arguments against their own existence

Key Arguments #

Against Light as Subtle Bodies #

Argument 1: Spatial Occupancy

• Two bodies cannot be in the same place
• If light were subtle bodies spreading through air, both light and air would occupy the same space
• Therefore, light is not subtle bodies

Argument 2: Temporality of Motion

• All locomotion requires time
• Illumination (especially across vast distances like east to west) appears to take no time
• Therefore, illumination is not locomotion
• Objection addressed: One might say light moves so fast we cannot detect the time. Aristotle replies: moving from east to west is a very great distance; even if we miss small distances, we should notice this vast distance.

For Light as Quality/Form #

• Light actualizes the transparent, just as fire or lucid body makes transparent potential become transparent actual
• Darkness, not being a body, is the lack or privation of light
• Since darkness is not a body, light (its contrary) is similarly not a body but a form or quality
• This explains why color cannot be seen without light: the transparent must be actually transparent (illuminated) for color to move it

Important Definitions #

τὸ Διαφανές (The Transparent) #

That which is visible not in virtue of itself but through an extraneous color; capable of being in potency (dark) or in act (illuminated).

Φῶς (Light) #

The actuality of the transparent; a form or quality of the transparent, not a body or stream of particles; the presence of fire or lucid body in the transparent.

Χρῶμα (Color) #

That which moves the transparent in act; cannot be seen without light; requires a transparent medium between the colored object and the eye.

Examples & Illustrations #

Visibility and the Medium #

• Colored object on the eye: Placing a colored object directly on the eye prevents vision; the transparent medium (air) between the eye and object is necessary
• Illumination of a room: Happens instantaneously, unlike the slow spread of particles, suggesting light is not locomotion
• Fungi and fish eyes: Visible in darkness (self-luminous things like fungi and fish eyes with reflective properties) show that not all visible things depend on external light

Understanding Through Magnification #

• Definition as magnification: “This is a square” expands to “an equilateral and right-angled quadrilateral,” allowing clearer and more distinct understanding
• Socrates in the Republic: To truly convince his interlocutors that the just man is happier than the unjust, Socrates magnifies the human soul by examining it in the city-state, where the structure is “blown up” so large that one cannot miss it
• Proportional analogies: “Reason is to the emotions as a man is to a horse” makes intellectual truths visible by comparing them to physical examples more easily perceived

Modern Experiments #

• Wave vs. particle debate: Newton and Huygens disagreed; 19th-century diffraction experiments favored waves for ~200 years
• Photoelectric effect: Einstein showed this made sense only if light were particles (photons), reviving corpuscular theory
• Michelson-Morley experiment: Enabled measurement of light’s speed (~186,000 miles per second), showing Aristotle’s premise about instantaneous illumination was incorrect

The Importance of Instruments and Tools #

• Telescope and microscope: Extended the eye’s range and magnifying power for sensory observation
• Definition and demonstration: Extended reason’s range, allowing it to know distant truths through logical chains
• Moderns’ neglect: Abandoning definition and demonstration as tools, modern philosophers miss important insights (though they gain from telescopes and microscopes)

Notable Quotes #

“Light, however, is like the color of the transparent, whenever the transparent is an actuality due to fire or something of this sort.” — Aristotle (cited in De Anima II.7, 418b)

“For two bodies, you’re not able to be together in the same place.” — Berquist, explaining Aristotle’s rejection of light as a body

“The sense being like a certain mean of the contrarieties and sensibles. And because of this, it discerns the sensibles. For the middle is able to discern.” — Aristotle (cited by Berquist, regarding how the sense organ acts as a mean)

Questions Addressed #

Is light a body or a quality? #

• Aristotle argues it is a quality or form of the transparent, actualized by fire or a lucid body
• Two arguments support this: the impossibility of two bodies in the same place, and the instantaneous nature of illumination
• Modern physics (photons) has returned to a corpuscular view, yet Aristotle’s reasoning remains philosophically sound

Why does illumination appear instantaneous? #

• If light were locomotion (movement of bodies), it would take time proportional to distance
• The illumination of vast distances (east to west) appears to take no time
• Therefore, illumination is not locomotion but the actualization of a form (light in the transparent)

How do we reconcile Aristotle’s view with modern physics? #

• Aristotle’s arguments are valid but based on the false premise that illumination truly takes no time
• Modern measurement (~186,000 miles per second) shows it does take time, but imperceptibly to human observation
• The ancient Greek approach was reasonable: extend common experience as far as possible before requiring specialized knowledge
• The modern photon theory is closer empirically to Empedocles than to Aristotle, yet Aristotle’s philosophical reasoning about why light cannot be a body was genuinely compelling given the available evidence