Book: Physics

Overview

Aristotle’s Physics is a foundational inquiry into nature, change, and the principles that govern the physical world. It asks what natural things are, how they come to be, what counts as a satisfactory explanation, and why motion and time exist at all. Throughout, Aristotle develops a vocabulary, cause, nature, potentiality, actuality, that becomes the scaffold of later science and philosophy.

Nature and the Four Causes

Nature (physis) is defined as an internal principle of motion and rest. Stones, animals, and plants possess sources of change within themselves, unlike artifacts whose form is imposed from outside. Understanding a thing requires grasping four kinds of cause: material (what it is made of), formal (its defining account or structure), efficient (the source of change), and final (the end or purpose for the sake of which it is). Natural explanation is therefore teleological: events in nature are intelligible because they are ordered toward ends, even when material and efficient factors are also necessary.

Change, Motion, Potentiality, Actuality

Change is analyzed as movement from potentiality to actuality. Motion (kinesis) is the actuality of the potential qua potential: the unfolding of a capacity precisely as it is being realized. Aristotle distinguishes kinds of change, generation and corruption, alteration of qualities, growth and diminution, and locomotion, arguing that locomotion is primary because the others presuppose place and contact. The doctrines of potentiality and actuality explain how something can be capable of contrary outcomes, why change is neither instantaneous nor arbitrary, and how continuity is possible without positing infinite completed states.

Place, Void, and Time

Place (topos) is not an independent container but the innermost boundary of the containing body that is at rest relative to what it contains. This account allows bodies to change place without invoking a separate receptacle. On this basis Aristotle denies the existence of a void: motion does not require empty space, and a void would generate paradoxes about speed and resistance. Time is the number of motion according to the before and after. It depends on motion for its structure and on a soul to count, yet it is not merely subjective, since motion possesses an ordered before-and-after whether or not anyone counts. The analysis accounts for time’s divisibility, its continuity, and the puzzle of the now as both boundary and part of temporal flow.

The Continuum and the Infinite

Bodies, magnitudes, and time form continua. By carefully distinguishing between actual and potential infinite, Aristotle defends the thesis that infinity exists only potentially, as unending divisibility or indefinite extension of counting, never as a completed magnitude. This preserves mathematics and physics from contradictions while explaining how motion and measurement proceed without gaps.

Chance, Spontaneity, and Necessity

Events sometimes occur by chance or spontaneity, but these are incidental causes that presuppose teleology. Chance arises chiefly in contexts of purposive action when independent causal lines intersect; spontaneity belongs more broadly to natural processes lacking deliberation. Material necessity constrains what ends can be realized, yet final causes are explanatory primaries: the heart has such-and-such structure because animals must live in such-and-such way.

Eternal Motion and the First Mover

Aristotle argues that motion is eternal and that there must be a first unmoved mover as the ultimate explanatory principle of continuous celestial motion. This mover acts not by pushing but as an object of desire and thought, drawing motion as a final cause. The cosmos thereby exhibits order without needing a temporal beginning, and natural motion, toward an end, in a plenum, without a void, becomes intelligible within a unified framework of causes, natures, and forms.