Themes
Change, Motion, and the Structure of Nature
In Physics, Aristotle sets out his ambition to explain the nature of change. Earlier philosophers—many of whom he mentions by name—had attempted to reconcile the reality of motion with logical arguments, with some suggesting that change is impossible. Aristotle addresses this challenge by developing a detailed philosophical account of change, motion, and the structure of nature.
Aristotle begins by defining nature as a principle of motion and rest within things themselves. Natural objects differ, he says, from artificial objects because they possess an internal source of change. A tree grows and develops according to its own nature, whereas a bed or a statue depends on external craftsmanship. Understanding nature, therefore, requires an understanding of the principles that explain how natural things change and move.
To analyze change, Aristotle introduces the concepts of potentiality and actuality. Potentiality refers to the capacity for a certain state, while actuality refers to the realization of that capacity. Change occurs when something that exists potentially becomes actual. A block of marble, for example, has the potential to become a statue. This potential becomes actual through the sculpting process. Aristotle further explains change through the interaction of matter, form, and privation. Matter is the underlying substance that persists through transformation. Form is the structure or condition that defines a thing. Privation refers to the absence of that form prior to the change.
When change occurs, Aristotle says, matter acquires a new form while losing the previous state of privation. These ideas allow Aristotle to address the Eleatic claim that change is impossible. He responds to thinkers such as Parmenides, who claimed change would require something to come from non-being. Aristotle disagrees. Aristotle believes that, rather than absolute non-being, change involves the transition from potentiality to actuality within an existing substance.
Aristotle also classifies different kinds of change, which he divides into four main types. Generation and destruction involve the coming into being or passing away of substances. Alteration involves qualitative changes such as heating or cooling. Increase and decrease involve changes in quantity, such as growth or shrinking. Change of place involves motion through space. Between these varieties of change, Aristotle considers movement in place to be the most fundamental. Other forms of change depend on spatial relationships between objects. Heating, for example, requires a heat source to come into contact with the object being heated. Aristotle suggests that because such interactions involve spatial motion, movement in place underlies many other natural processes.
Aristotle also addresses paradoxes about infinite divisibility. Zeno argued that motion is impossible because a moving object must traverse infinitely many divisions of space. Aristotle distinguishes between potential and actual infinity. A line is infinitely divisible in potential, but it does not contain infinitely many divisions as actual parts. Motion, therefore, does not require completing infinitely many tasks. Similarly, continuity plays an important role in Physics. Motion occurs within a continuous medium that can be divided indefinitely, but is not composed of discrete indivisible units.
This reasoning allows Aristotle to explain how movement can occur smoothly, without requiring objects to jump between separate points in space. Aristotle also connects motion to time. He defines time as the number of motion with respect to before and after. Time, in this way, is a measure that arises from observing change. Without motion or change, there would be no way to count temporal intervals. Therefore, time depends on motion rather than existing independently.
Infinity and Continuity in Aristotle’s Philosophy of Time
The concepts of time and infinity play a crucial role in Aristotle’s explanation of motion and change. Earlier philosophers had encountered serious difficulties with regards to infinite divisibility and the structure of time, particularly through Zeno’s paradoxes. In Physics, Aristotle reviews existing notions of infinity and continuity in order to develop his own account of continuity and the nature of the infinite.
Aristotle rejects the idea that the infinite exists as a completed or actual quantity in nature, as was suggested by earlier philosophers. He argues instead that infinity exists only potentially. A magnitude such as a line can be divided indefinitely, but this divisibility does not imply that infinitely many divisions exist simultaneously. Each division occurs only when the line is actually divided. This distinction between potential and actual infinity allows Aristotle to resolve paradoxes about motion. Zeno’s arguments rely on the assumption that a moving object must traverse infinitely many actual segments of space. Aristotle responds that these divisions exist only potentially within the continuum. A moving object passes through them without requiring each division to exist as a separate stage.
Aristotle’s understanding of the continuum is part of his broader understanding of the nature of time, particularly with regards to his developing theories of natural philosophy. Time is not composed of indivisible moments in the way that points compose a line. Instead, time is continuous and infinitely divisible. The present moment—which he refers to as the “now”—functions as a boundary between past and future, rather than as a discrete unit of time. Aristotle argues that time depends on motion, defining time as the number of motion with respect to before and after.
The logical consequence of this decision, Aristotle posits, is that time arises from the ordering of events within processes of change. If there were no motion or change, there would be no basis for distinguishing earlier from later. This conception of time also explains why time appears to flow continuously: Because motion occurs within a continuous magnitude, the measurement of motion through counting produces a continuous temporal sequence. Time therefore reflects the structure of change in the natural world.
Aristotle’s treatment of time also addresses philosophical problems concerning temporal boundaries. The present moment is both the end of the past and the beginning of the future. Although the same instant can be described in both ways, it does not fully belong to either temporal interval. Instead, it functions as the dividing point that connects them. The relationship between time and motion leads Aristotle to eventual conclusions about the eternity of the universe. Since time is inseparable from motion—and because Aristotle argues that motion must always exist—time must also be eternal. There can be no moment before time began or after time ends.
As a result, Aristotle’s interpretation of infinity and time supports his broader philosophy of nature. By treating infinity as a potential property of magnitudes and by defining time as a measure of motion, Aristotle provides an explanation of continuous change. His analysis resolves paradoxes that had previously cast doubt on the possibility of motion and establishes a framework for understanding the temporal structure of the natural world.
The Unmoved Mover and Eternal Motion
Aristotle’s conception of the unmoved mover and eternal motion has become one of his most influential ideas. The concept concerns the existence of a first unmoved mover, an idea which Aristotle develops while investigating whether motion in the universe is eternal. He attempts to logically explain the ways in which continuous motion can be understood.
Aristotle begins by observing that everything in motion appears to be moved by something else. A stone moves because it is pushed or thrown, or a cart moves because it is pulled by an animal. An animal, he writes, moves because of internal processes that may be hidden from the observer which, he believes, do exist. This observation suggests that motion involves causal relationships between movers and moved objects. If every moving thing were moved by another moving thing, the chain of causes would extend indefinitely. Aristotle argues that such an infinite regress cannot explain motion. Without a first cause, the sequence of movers would never begin. Therefore, there must be a primary source of motion that initiates movement without itself being moved by another. Aristotle calls this source the “unmoved mover.”
Unlike ordinary movers—which both move and are moved—the unmoved mover produces motion while remaining unchanged. This principle provides what Aristotle believes to be the ultimate explanation for the existence of motion in the universe. Building on his idea, Aristotle reasons that motion must be eternal. If motion had begun at some moment in the past, there would have been a prior state in which objects had the capacity to move but were not moving. Explaining the transition from rest to motion would, therefore, require another motion. This leads to a contradiction. Aristotle therefore believes that motion must always have existed and—because motion is eternal—the cause of motion must also be eternal. The unmoved mover cannot be something that comes into existence or ceases to exist. Instead, it must exist permanently as the source of continuous motion in the cosmos.
Aristotle also sets out to explain the type of motion that can exist eternally. Between the different kinds of motion, only circular motion can be continuous and without interruption. Motion along a straight line eventually reaches an endpoint or reverses direction, whereas circular motion can continue indefinitely, since its starting point and endpoint coincide. The eternal circular motion of the heavens therefore provides the clearest example of continuous motion in the natural world. Aristotle concludes that the unmoved mover is responsible for sustaining this motion. Since the heavenly spheres move eternally in circular paths, he believes, the cause of their motion must be a stable and unchanging principle.
Aristotle also argues that the unmoved mover cannot possess physical magnitude. A finite magnitude cannot generate motion indefinitely, so an infinite magnitude cannot exist according to Aristotle’s own theory of nature. The result of this is that the first mover must be indivisible and without physical extension. Through the argument for the unmoved mover, Aristotle provides an explanation for the existence of motion in the universe. Motion is eternal, according to Aristotle, because it depends on a source that is itself free from change.
Unlock every key theme and why it matters
Get in-depth breakdowns of the book’s main ideas and how they connect and evolve.
- Explore how themes develop throughout the text
- Connect themes to characters, events, and symbols
- Support essays and discussions with thematic evidence