Aristotle's Physics: Change, Motion, Time, and Causation

Imagine a mug of coffee sitting on a desk. An hour later, the coffee is cooler. It is tempting to say that we have already explained the event once we have named its beginning and end. But that leaves several questions unanswered. What underwent the change? Which capacity made that change possible? What brought it about? And what distinguishes the cooling process from simply being cold?

Aristotle's Physics investigates the principles needed to answer questions like these. Its subject includes growth and qualitative alteration as well as movement from one place to another. It also asks about the conditions under which change can occur: place, continuity, and time. The work joins questions about explanation to claims about the physical world; those claims need to be assessed along with its concepts.

The central proposal is that change can be understood without imagining that something emerges from sheer nothing. Something capable of being otherwise undergoes a determinate process. Yet a capacity, an ongoing change, and a completed result are different conditions. Keeping them distinct helps explain why Aristotle describes motion through both potentiality and actuality, rather than treating those terms as simple opposites.

In Book I, Aristotle examines arguments that make coming-to-be appear impossible. If something comes from what already is, why call it coming-to-be? If it comes from what is not, how can nothing produce something? His response distinguishes the underlying subject from the character it acquires and the character it previously lacked (Physics I.7–8).

His example is a person becoming musical. The person persists, but being unmusical does not persist as the resulting condition. We can therefore distinguish what undergoes the change from the description under which it changes. The musical person comes from someone who was not musical, without coming from absolute nothing. A determinate lack is different from the absence of any subject (Physics I.7–9).

This needs qualification when we consider substantial generation rather than a change in someone's skill. A new living thing does not simply mean that an already complete individual has acquired another incidental property. Aristotle's inquiry must explain coming-to-be as well as alteration. The distinction between an underlying material basis and an acquired form supplies his framework for addressing these cases, without implying that the same organized individual survives every kind of change.

For a contemporary analogy, consider clay being made into a bowl. The clay is not nothing before the bowl exists, but neither is it already an actual bowl. Saying it is capable of becoming a bowl identifies a possibility grounded in the material and the circumstances. It does not imply that every possible object is already hidden inside the clay, waiting to be uncovered.

Book II begins by distinguishing things that exist by nature from products of craft. Animals, plants, and the elementary bodies have an internal source of change and rest. A bed, considered specifically as a bed, does not. Its wood can nevertheless have natural capacities, so the distinction depends on the respect in which we describe the thing (Physics II.1).

A wooden chair may decay. That does not show that its furniture design possesses its own natural life cycle in Aristotle's sense. The material changes because of the capacities of the wood. Conversely, a plant's growth belongs to the kind of living thing it is. This is an illustration of Aristotle's distinction, not a claim that he analysed chairs using our manufacturing methods.

An internal principle is not the same as complete independence from external conditions. A plant can require water and suitable surroundings. Nor does the definition say that every natural body consciously directs itself. The wording includes being moved and being at rest. Reading "internal" as "needs nothing else" would make the distinction less informative.

Aristotle considers both matter and form as candidates for nature, and gives form priority in an important respect: what has reached its form is more fully the thing in question than what has it only potentially. His example contrasts the material of a bed with the organized living thing that generates another living thing of its kind (Physics II.1). Nature therefore concerns organization as well as material composition.

The student of nature must investigate both. A definition of an organism cannot ignore its material realization, while a list of ingredients alone may fail to explain what makes it the organized thing it is. Aristotle compares the required knowledge with a physician's knowledge of health and the bodily materials in which health is realized (Physics II.2).

In Physics II.3, Aristotle distinguishes four senses of cause. The Greek term translated as cause has an explanatory range broader than the familiar question of which event immediately triggered another. The four senses identify different answers to a request for explanation:

1. The material cause is that out of which something comes to be and which remains in it, such as bronze in a statue.
2. The formal cause is the defining account or structure, what makes the thing the kind of thing it is.
3. The source of change, called the efficient cause here, identifies what initiates the relevant making or changing.
4. The final cause is that for the sake of which something happens, such as health explaining why someone takes a walk.

The same thing can have several causes in these different senses. Bronze does not compete with sculptural expertise as an explanation of a statue. They answer different questions. Similarly, the goal of health can explain the decision to exercise without becoming an additional material component of the body (Physics II.3).

Imagine a bowl commissioned for serving soup. Clay answers the material question. The organized shape that makes it a bowl answers the formal question. The potter's skilled activity helps answer the question about its production. Holding soup supplies the stipulated purpose. This is an author-supplied example of an artifact, where an intention makes the final-cause explanation comparatively straightforward.

These should not be pictured as four separate objects attached to every event. In natural generation, Aristotle says the form, the end, and the source of motion often converge: the mature form is an end of development, while another organism of the same form initiates generation (Physics II.7). We still need to identify the explanatory role under discussion, rather than count physically distinct causes.

The framework also distinguishes an intended explanation from a coincidental result. In the discussion of chance, Aristotle considers a person who encounters someone who owes him money and thereby collects a debt, although collecting it was not why he went to that place. The encounter can fall within a purposive activity without itself having been arranged for that purpose (Physics II.4–6). Calling an outcome useful does not establish that it was what the process was for.

Aristotle's natural teleology claims that some natural processes occur for the sake of ends. This does not mean that a plant thinks about what it wants to become. In II.8 he argues that the absence of deliberation does not rule out end-directed activity. His comparison with craft concerns the organization of a process toward a result, not a conscious plan inside every growing thing.

He presents a serious alternative. Perhaps useful arrangements arise through material necessity, and the arrangements that happen to work persist. The example concerns teeth: sharp front teeth tear food while broad molars grind it. Why not regard their usefulness as a fortunate result rather than their explanatory end? Aristotle answers by appealing to the regularity of such natural arrangements and their organized development (Physics II.8).

A reader can dispute that inference. Regular outcomes might require explanation without establishing the particular final-cause account Aristotle favours. The alternative he presents is worth reading carefully, but we should not equate its brief ancient formulation with a complete modern evolutionary theory. The chapter can identify the explanatory issue without claiming that Aristotle anticipated or refuted a later science.

Material necessity still matters in Aristotle's own account. In II.9 he asks what conditions are necessary if an end is to be realized. A saw must have appropriate material if it is to perform its work. Specifying what the tool is for does not make any material adequate. Conversely, listing its material properties does not by itself specify its function. The point is the relation between an end and the conditions required for it, not permission to ignore mechanisms.

In III.1, Aristotle defines motion through the actuality of what is potential, precisely insofar as it is potential. The final qualification prevents the definition from collapsing the process into its completed result. What is buildable is actualized as buildable while it is being built. When the house is finished, the same building process is over (Physics III.1–2).

Consider water being heated. Before heating, it may be capable of becoming warmer without undergoing that change. While heating, it is actually undergoing a process toward a warmer condition. When the target condition has been reached and heating ceases, that particular warming process has ended. This illustration separates the capacity, its ongoing realization, and the stipulated completion without claiming that water has one natural target temperature.

Aristotle also argues that the mover's action and the moved thing's undergoing can be one process described differently. Teaching and learning illustrate how an activity can be attributed to an agent and to the recipient without becoming two disconnected changes (Physics III.3). That analysis raises questions about how to locate a process; it does not make teacher and learner the same person.

The discussion of motion leads Aristotle to the infinite. He rejects an actually infinite bodily magnitude while allowing an infinity of further division. A finite length can always be divided again; we need not suppose that an infinite collection of bodily parts has been completed as a separate object. The relevant infinity concerns the inexhaustibility of the procedure (Physics III.6–7).

In Book VI, he connects the divisibility of distance, motion, and time. A continuous line is not assembled out of indivisible points as if points were tiny lengths. Points mark boundaries, while a segment can be divided into further segments. Correspondingly, the divisibility of a journey does not alone show that the journey needs an unlimited duration (Physics VI.1–2).

This bears on the argument attributed to Zeno that a traveller must first traverse half the route, then another division, and so on. Aristotle distinguishes an infinity in respect of divisibility from an infinite extent. A finite time is also divisible, so infinitely many possible divisions of a route do not by themselves preclude its traversal in a finite time (Physics VI.2, VI.9). This is his response within a theory of continuity, not a presentation of modern calculus.

Book IV asks what a body's place is. Aristotle distinguishes it from the body's own matter or form and from a separately existing spatial interval. His answer identifies place with the first unmoving boundary of what contains the body (Physics IV.4). Both "boundary" and "unmoving" matter.

A vessel provides a first illustration of containment. Aristotle explicitly distinguishes a vessel, which can be carried, from place as an immovable container. His river example makes the difficulty visible: the immediately surrounding water can flow past a boat, while a broader containing arrangement supplies the fixed reference. Removing the unmoving qualification turns his account into a different one.

The account has substantial physical commitments. Aristotle rejects a separately existing void and argues against the idea that locomotion requires empty gaps. Bodies can make room for one another. He also considers how different media affect speed and argues that motion in a void would create problems for the ratios he uses to compare movement (Physics IV.6–9).

We can assess the reasoning without insulating it from physical criticism. His assumptions about motion and resisting media do not become correct merely because they appear in a philosophical inquiry. A philosophical interest in place does not require defending all the physical conclusions he draws from it. The task is to distinguish which assumptions an argument uses and whether its conclusion follows from adequate assumptions.

Aristotle describes time as the number of motion in respect of before and after. He distinguishes time from motion itself: movements can be faster or slower, whereas time provides the measure by which such differences are judged. We recognize an interval by distinguishing an earlier and a later phase of change (Physics IV.10–11).

Here, number concerns what is counted or countable. A clock offers a contemporary illustration: a regular process supplies distinguishable intervals with which other processes can be compared. The example does not show that time is identical with this clock's particular movement. Aristotle likewise distinguishes the measurement relationship from identifying time with one motion (Physics IV.11–12).

His account includes a difficulty about the soul. If counting requires a soul capable of counting, would time exist without soul? In IV.14 he distinguishes motion and its before-and-after structure from that structure as countable. The passage raises a question about time's dependence on a counter; it should not be flattened into either "time is only in the mind" or an unqualified claim that counting is irrelevant.

This distinction also limits easy appeals to subjective experience. A meeting may feel long, but its measured duration and that feeling are different questions. Aristotle's account here concerns ordered, countable change. Treating it as a direct explanation of boredom or attention would add a psychological theory the passage does not supply.

Book VIII asks whether motion began and whether it can cease altogether. Aristotle argues for everlasting motion: a proposed first change seems to require prior conditions whose coming-to-be or readiness would themselves require explanation. His conclusion is not a universe that might begin while time exists only abstractly without a beginning. The argument defends the continuity and eternity of motion (Physics VIII.1).

He then investigates the dependence of what is moved on a mover. Even a self-moving animal requires a distinction between what moves and what is moved within the organized whole. In developing the account of continuous motion, he argues for an unmoved source. The final chapter denies that this first mover has bodily magnitude (Physics VIII.4–6, VIII.10).

The argument examines the dependence of motion. It distinguishes something that transmits motion while being moved from a source that does not undergo that dependence. A claim specifically about moved things has a narrower scope than a claim about everything that exists. Whether the physical and explanatory premises succeed remains open to criticism. Summarizing the conclusion as obvious would conceal the work its premises must do.

The familiar account of a mover as an object of desire belongs to the discussion in Metaphysics XII.7. It should not silently replace the argument of Physics VIII. Comparing the works can deepen the question of how an unmoved source causes movement, but the comparison must preserve what each text argues.

For contemporary work, the four causes can be used as an explicitly modern set of questions. Suppose a team is explaining a failed product prototype. Material and component constraints differ from its organization, the process that produced it, and the purpose it was meant to serve. Distinguishing these questions may expose an explanation that answers only one of them. It does not prove that the artifact possesses a natural end in Aristotle's sense.

Potentiality offers another limited application. A learner's capacity is not the same as the practice through which it develops or the skill eventually achieved. That distinction can clarify a discussion of development without supplying a fixed ideal for every person's life. The Physics does not decide which ambitions someone ought to choose merely by describing change.

The work also teaches a habit of separating an intelligible question from a successful answer. Asking what makes a process continuous does not establish every ancient theory of motion. Asking what a structure is for does not establish that every useful result was intended. Aristotle's explanations deserve assessment at the level of their particular claims. That is how the investigation of change remains an investigation, rather than becoming a vocabulary that substitutes for one.

• Aristotle, Physics I and II, for the principles of change, nature, and the four explanatory causes. The linked translation is by R. P. Hardie and R. K. Gaye.
• Aristotle, Physics IV.14 and VIII.10, for the soul-and-time question and the conclusion about the first mover's magnitude.
• Aristotle, Metaphysics XII, especially chapter 7, to compare the argument about desire and an unmoved mover.
• Aristotle, On Generation and Corruption, for a further inquiry into coming-to-be and qualitative change.