Aristotle’s Physics: Matter, Form, Change, and Cosmos

Posted on Mar 31, 2025 in Philosophy and ethics

Aristotle’s Concept of Nature (Physis)

For Aristotle, physics is the study of nature (physis). Physis refers to that which has within itself a principle of motion and stationariness, as opposed to things made by humans or existing by divine intervention.

Hylomorphism: Matter and Form

Aristotle rejects Plato’s Theory of Forms, particularly the idea that forms exist separately from physical objects. Instead, Aristotle conceives the universal (form) as intrinsically linked to the physical world. Hylomorphism is the theory that every physical object consists of both matter (hyle) and form (morphe).

  • Matter (Hyle): The underlying substrate that composes physical things. It is indeterminate and eternal. The simplest physical bodies are composed of the four elements: water, air, earth, and fire. All other complex bodies are composed of these. Matter represents pure potentiality and is the principle of individuation.
  • Form (Morphe): The essence or ‘whatness’ of a thing. Forms are inherent in things themselves. Each natural object possesses a form that makes it what it is. For Aristotle, in humans, the soul is the form.

Aristotle distinguishes between:

  1. Primary Substance: The individual composite of matter and form (e.g., this specific human).
  2. Secondary Substance: The universal form or species/genus (e.g., humanity).

Motion and Change (Kinesis)

Aristotle’s physics focuses on living beings (and natural objects) which possess an internal principle of movement or change (kinesis). He defines movement by contrasting being and non-being. There are two types of non-being:

  1. Absolute Non-Being: That which simply is not.
  2. Relative Non-Being: That which is not currently, but has the potential to be (potentiality).

Movement is possible because potential non-being can become being. Aristotle calls the potential state potentiality (dynamis) and the realized state actuality (energeia or entelecheia). Therefore, movement or change can be defined as the transition from potentiality to actuality.

The Four Causes of Change

Every change or movement involves four causes:

  1. Material Cause: The matter out of which something is made or changes (e.g., the bronze of a statue).
  2. Formal Cause: The form, essence, or pattern towards which the change tends; the new form actualized from potentiality (e.g., the sculptor’s idea of the statue).
  3. Efficient Cause: The agent or principle responsible for initiating the change; the immediate factor causing the change to occur (e.g., the sculptor).
  4. Final Cause (Telos): The end, purpose, or goal toward which the change is directed (e.g., the purpose of the statue, perhaps commemoration or decoration).

Factors in Any Change

Three factors concur in any process of change:

  1. Form: The form that the hylomorphic compound attains through the process of change.
  2. Privation: The absence of the form that will be actualized; that which exists in potentiality but does not yet actually exist before the change is complete.
  3. Substrate: That which remains constant throughout the change; the underlying matter.

Time and Space

  • Time: For Aristotle, time results from movement. It requires a ‘before’ and an ‘after’, making it measurable and uniform. Consequently, understanding time requires understanding movement. Since movement characterizes physis, the fundamental concepts of nature precede any understanding of time.
  • Space: Aristotle did not conceive of space as infinite void. The cosmos is finite and limited. For Aristotle, a body ends where another body begins; there is no vacuum. Space is understood as the place (topos) occupied by a body.

Aristotelian Cosmology

The universe, according to Aristotle, is finite, spherical, and eternal, consisting of concentric spheres. He divides the universe into two distinct regions:

  1. Sublunar Region: The area beneath the sphere of the Moon. Everything in this region is subject to all forms of change (generation, corruption, alteration, locomotion) and is composed of the four elements: earth, water, air, and fire. These elements have natural motions (earth and water downwards, air and fire upwards).
  2. Supralunar Region: The area from the Moon’s sphere outwards. This region consists of concentric spheres, each fixed with celestial bodies (stars, planets). These spheres move with perfect, eternal circular motion. The celestial bodies are composed of a fifth element, aether, which is incorruptible and unchanging.

Types of Celestial Bodies

  1. Fixed Stars: Located on the outermost sphere, moving together, constituting the limit of the cosmos.
  2. Planets (‘Wandering Stars’): Each belonging to its own sphere(s), exhibiting more complex movements.
  3. Sun and Moon: Also moved by a number of spheres.

The Prime Mover (Unmoved Mover)

To explain the eternal motion of the celestial spheres, Aristotle posits a Prime Mover, often identified with God. This entity is:

  • An immobile substance that does not belong to the physical world.
  • Non-spatial, eternal, and simple.
  • Not composed of matter and form, nor potentiality and actuality; it is pure actuality (actus purus).
  • A pure, perfect, and self-sufficient activity.
  • The only activity that involves no change is thought. Therefore, the Prime Mover’s activity is perfect thought, thinking about itself (thought thinking thought).

The Prime Mover initiates motion not through physical contact, but as a final cause. It is located conceptually at the periphery of the universe, and its perfection inspires the eternal circular motion of the first celestial sphere, which in turn transmits motion to the other spheres.