Aristotle's Physics: Motion, Change, and the Cosmos

Aristotle's Physics: The Nature of Being and Motion

Aristotle attempted to overcome the gulf that Plato had introduced between the intelligible world and the sensible world. This separation was unmaintainable for Aristotle.

Like Heraclitus, Aristotle believed that natural beings are subject to constant movement. The very condition of physical beings is to be mobile. Nature, for Aristotle, is a principle of movement and change. To think of the physical world is to think of it as becoming—a continuous transformation that responds to a reason or logos.

The Principle of Motion and Change

Aristotle rejects an external principle of motion for things and is compelled, in a sense, to admit that 'not-being' exists within the real, since any change is a process of becoming something that was not. Becoming implies the prior existence of something that exists in potentiality but not in actuality. This pre-existence of potential is the beginning of movement.

Regarding movement in beings, we can make two considerations:

• Actuality (Energeia): This refers to what something is at a given moment, here and now. It is a concept that indicates both the process of realization and its completion.
• Potentiality (Dynamis): This is understood as the capacity to become. It is what something is not yet, but can be.

Any change or movement can be understood as the transition from potentiality to actuality, as the culmination of a being that pre-existed in potential.

Types of Change

If the process involves humans, we must distinguish between:

• Artificial change or movement.
• Natural change or movement.

Natural change can occur in two ways:

• Accidental Change: This involves a variation in the attributes (accidents) of a substance, without the substance itself being affected. There are three possibilities for accidental change:
  • Quantitative
  • Qualitative
  • Local (change of place)
• Substantial Change: This occurs when the substance itself changes.

Aristotelian Cosmology and the Unmoved Mover

The analysis of motion requires clarifying the concepts of place and time. Regarding time, it is a measure of motion. Ultimately, all motion originates from something that is not mobile: the Unmoved Mover.

Regarding place, it is neither form nor matter but is inherent to an entity; it is the immediate and immobile limit of beings. Place does not exist separately from things. An absolute and independent space is inconceivable. A vacuum (empty space) is impossible, as a place without bodies to limit it is unthinkable. Finally, space is not homogeneous; a suitable place exists for each thing.

Cosmological Model

Aristotle divides the universe, distinguishing between:

• The Sublunar World: Composed of the four elements (earth, water, air, fire), where local movements occur.
• The Supralunar World: Composed of ether and subject to perfect circular motion.

Aristotle's cosmological model is spherical, and its geometric center is the Earth. The universe is finite and limited, subject to uniform circular motion. It is a compact, multiple, and heterogeneous sphere consisting of concentric spheres, culminating in the sphere of the fixed stars.