Aristotle’s Physics (4th century BCE) is a foundational natural-philosophy treatise that sets out to explain change (kinesis) by identifying the principles and causes at work in natural things, rather than exploring “physics” in the modern, quantitative sense. Across eight books, Physics defines nature, motion, and infinity. Aristotle analyzes place, void, and time as preconditions of movement, while also arguing for continuity against paradoxes of motion. Key themes include Change, Motion, and the Structure of Nature, Infinity and Continuity in Aristotle’s Philosophy of Time, and The Unmoved Mover and Eternal Motion.

This guide uses the 2008 edition of Physics, translated into English by Robin Waterfield.

Summary

Physics begins with the methodological claim that the study of nature requires identifying the principles that explain natural things. Aristotle argues that natural inquiry should proceed from what is more familiar to human perception toward the underlying causes that explain phenomena. Nature, he argues, is the internal principle responsible for motion and rest in natural bodies. Unlike artificial objects—which depend on external makers—natural things possess their own internal source of development and change.

In Physics, an important early argument concerns the principles underlying change. Aristotle identifies three fundamental components involved whenever something changes: Matter, form, and privation. Matter is the underlying substratum that persists through change; form is the structure or state achieved by the object; and privation is the absence of that form prior to the change. When a bronze statue is produced, for example, the bronze is the matter and the statue’s shape is the form, while the absence of that form beforehand is the privation. This framework allows Aristotle to explain how change can occur without requiring something to come from absolute non-being.

From this foundation, Aristotle introduces one of his most central ideas: The doctrine of the four causes. To explain any natural phenomenon fully, he argues, a person must identify four kinds of explanatory factors: The material cause refers to what something is made of; the formal cause is the structure or defining nature of the thing; the efficient cause is the source of motion or change; and the final cause is the purpose or end toward which the process is directed. Aristotle argues that natural processes are intelligible only when all four types of explanation are considered together. This approach contrasts with earlier thinkers, who attempted to explain nature solely through material or mechanical causes.

Aristotle also addresses the roles of chance and necessity in natural processes. He argues that chance events occur when outcomes arise from processes that normally serve a different purpose. Someone digging for a plant, for example, might accidentally uncover buried treasure. The discovery is accidental because it is not the intended goal of the action. Aristotle maintains that chance does not undermine the teleological structure of nature. Instead, it presupposes it. Natural processes generally aim at certain ends and chance events arise when independent causal chains intersect.

Another major topic in the Physics is the nature of the infinite. Aristotle rejects the idea that an actual infinite magnitude exists in nature. Instead, he proposes that infinity exists only potentially. For example, a line can be divided indefinitely. This divisibility, however, does not imply that an infinite number of divisions exists all at once. Aristotle’s treatment of infinity plays a key role in his response to paradoxes about motion and continuity and closely connected to this discussion is Aristotle’s analysis of place, void, and time. Aristotle defines place as the inner boundary of the containing body surrounding an object. He rejects the existence of a void or empty space, arguing that motion requires a medium and that the concept of a vacuum leads to contradictions in the explanation of movement. Time, in Aristotle’s work, is not an independent substance. Instead, time is the measure by which change is counted and ordered.

Physics also engages with famous paradoxes about motion, particularly those attributed to Zeno of Elea. Zeno argued that motion is impossible because a moving object must traverse infinitely many divisions of space. Aristotle responds by distinguishing between potential and actual infinity. A moving object passes through infinitely many potential divisions, but these divisions are not actual separate stages requiring completion one by one. Motion remains possible because the continuum is infinitely divisible only in potential.

Aristotle then develops a detailed theory of motion. He distinguishes several kinds of change: Change of substance (generation and destruction); change of quality (alteration); change of quantity (increase and decrease); and change of place (movement). Among these, movement in place is the most fundamental. Other forms of change presuppose spatial movement because processes such as heating, cooling, growth, and decay require interactions between objects that involve changes in position.

The analysis of motion leads Aristotle to consider whether motion is eternal. He argues that change must always have existed and cannot have begun at some moment in time. If motion had begun, he writes, there would have been a preceding state in which something had the capacity to move but did not yet do so, which itself would require a change to explain the transition. This reasoning leads Aristotle to conclude that motion is eternal.

From this premise, Aristotle develops what would become a core idea of ancient philosophy: The existence of a first unmoved mover. Aristotle observes that everything that moves is moved by something else. If each mover required another mover indefinitely, he suggests, the chain would never begin. Therefore, the series must terminate in a primary source of motion that moves other things without itself being moved. This first mover provides the ultimate explanation for motion in the universe. Aristotle further argues that the first mover must be indivisible and without magnitude. A finite magnitude cannot possess infinite power, he says, and an infinite magnitude is impossible according to Aristotle’s theory of nature. Therefore, the ultimate cause of motion must be immaterial and without physical extension.

Finally, Aristotle identifies the kind of motion produced by this first mover. Among all types of motion, circular motion alone can be continuous and eternal. Motion along a straight line must eventually reverse direction or stop, but circular motion has no fixed beginning or end. Since the starting point and endpoint coincide, circular motion can proceed indefinitely without interruption. Aristotle therefore concludes that the eternal motion of the heavens must be circular.