Aristotle responds to objections against his claim that change is eternal, claiming that these are “not difficult to deal with” (190). One objection states that no single change can be eternal because every change has a beginning and an end defined by opposites. Aristotle accepts that a single instance of change cannot last forever, but argues that a continuous kind of change can still be eternal.

A second objection claims that an object may begin to move even though it was previously at rest. Aristotle responds that this occurs because an external mover becomes present or active at a particular time. The strongest objection concerns living things that appear to begin moving without an external cause. Aristotle argues that this appearance is misleading. Internal bodily changes are always occurring, often caused by environmental conditions, and these changes stimulate thought or desire that leads to motion. Thus, movement never arises without preceding change.

Aristotle examines the question of why some things sometimes change and sometimes remain at rest. He considers several possibilities: Everything is always at rest; everything is always changing; some things always change while others always rest; or some things are capable of both change and rest.

He argues that the last option is correct. The claim that everything is always at rest contradicts clear sensory evidence and undermines all knowledge, since knowledge presupposes change. The opposite claim that everything is always changing is also false, because natural processes show clear limits and endpoints, such as health as the endpoint of recovery. Observations also show that bodies can remain at rest in their natural places. He also rejects the idea that some things always move while others always rest, since phenomena such as generation, destruction, and forced motion require objects that alternate between rest and change. Aristotle concludes that some things are capable of both change and rest, while others may be permanently in one state.

Aristotle argues that everything that changes is “changed by something” (195). An agent of change and an object of change may act or be acted upon either coincidentally or in their own right. Some things are changed by external agents, while others appear to move themselves, as animals do. In such cases, the movement is natural because the source of motion lies within the organism.

However, Aristotle explains that even apparent self-movement involves a distinction between the moving and the moved parts. Homogeneous bodies cannot move themselves because one part must act while another is affected. This also applies to natural motions such as the upward movement of fire or the downward movement of earth. These bodies are not self-movers, but move because their natural potentials are actualized when obstacles are removed. Light and heavy bodies move toward their proper places because their natures include tendencies in specific directions. When hindrances are removed, their potential becomes actual motion.

Aristotle concludes that every change occurs either by nature or by force and—in every case—something acts as the cause of the change.

Aristotle argues that if everything that changes is changed by something, the sequence of movers cannot continue indefinitely. In chains of motion, one agent moves another, as when a person moves a hand, the hand moves a stick, and the stick moves a stone. Since an infinite regress of such movers is impossible, there must be a first “agent of change” (200).

He considers whether this first mover is itself changed. If every mover were also moved, absurd consequences would follow, such as agents undergoing the same changes they produce or an endless sequence of different changes. Therefore, the chain must terminate in something that initiates motion without being changed by another.

Aristotle also analyzes self-motion. In a self-moving whole, one part acts as the mover while another part is moved. The moving part itself remains unchanged. From this, he concludes that the primary source of change must be an unmoved mover that initiates change without undergoing change itself.

Aristotle argues that because change is eternal, there must exist a first agent of change that is itself “eternal and unchanging” (207). Continuous change in the world cannot be explained by causes that arise and disappear over time, because such causes would not account for the necessary and uninterrupted continuity of change. Therefore, there must be a primary cause that persists eternally and remains unaffected by change, even coincidentally.

He maintains that, although some agents of change may exist only temporarily, they cannot account for the ongoing and necessary continuity of change. Instead, all changing processes ultimately depend on a fundamental source that exists apart from them and sustains their activity. This first agent is unique and eternal and it is preferable to assume a single primary cause rather than multiple or infinitely many causes.

Aristotle further argues that because eternal change must also be continuous, the cause responsible for it must likewise be singular and “stable” (211). If different agents were responsible at different times, the change would be successive rather than continuous. Thus, the existence of a single, primary, unchanging agent of change is necessary.

He also examines apparent self-movers such as animals. While animals appear to initiate movement, their motion depends on internal processes such as digestion, growth, and breathing, which in turn depend on external factors like food and environment. Therefore, their movement ultimately originates from causes external to themselves. Since these processes involve agents that are themselves changing, they cannot serve as the ultimate source of continuous motion.

Aristotle concludes that the first agent of change must be entirely unchanging, even accidentally. Only such an agent can guarantee the perpetual and stable continuation of change in the universe. This eternal source of motion explains why some things always change, while others change only intermittently depending on their relation to changing intermediaries.

Aristotle begins by asking whether any change can be “primary and continuous” (212) and, if so, which kind of change it is. Since change must be eternal, he says, the primary kind of change must also be continuous and eternal, and this is the type of change produced by the first agent of change.

He distinguishes three kinds of change: Change of size (increase and decrease); change of quality (alteration); and change of place (movement). Among these, movement is primary. Alteration presupposes movement because the agent that alters something must sometimes be nearer and sometimes farther from the object altered, which requires change of place. Likewise, processes such as condensation and rarefaction depend on combination and separation, and these involve movement. Increase and decrease also involve movement because parts are added or removed through spatial change.

Movement is also primary in several senses of priority. In terms of necessity, continuous change must exist and only movement can be continuous. In terms of time, eternal things cannot undergo other kinds of change, but they can undergo motion. In terms of form or completion, movement belongs to more fully developed beings, since organisms that attain completion acquire the capacity for motion.

Aristotle further argues that no other kind of change can be continuous and eternal. Changes such as generation, destruction, alteration, increase, and decrease occur between opposites. Since opposite states cannot exist simultaneously, there must always be an interval between such changes, preventing them from being continuous. Movement alone avoids this limitation. He concludes that movement, or change of place, is the primary and continuous form of change in nature.

Aristotle argues that “only circular movement” (216) can be continuous and eternal. Motion must occur either in a straight line, in a circle, or as a combination of the two. If straight motion cannot be continuous, then compound motions cannot be continuous either. Motion along a straight line cannot be continuous because it requires reversal of direction. When an object reverses direction, it moves between opposite directions and must stop at the turning point. This interruption introduces rest, which breaks continuity.

Continuous motion requires that movement proceed without such interruptions. Circular motion does not have this problem: In circular movement, the starting point and endpoint coincide, so the motion proceeds without reversal or stopping. Since it does not involve opposite movements or repeated halts, circular movement alone can be “infinite or continuous” (224).

Aristotle argues that circular movement is the primary kind of movement. All motion is either circular, straight, or a combination of the two. Since combined motion depends on the first two, circular and straight motion must be prior to it. Circular movement is superior because it is complete and capable of being eternal. Motion along a straight line cannot be infinite; even if it could exist, it could not be traversed. Movement on a finite straight line either reverses direction, making it composite, or stops, making it incomplete.

Circular motion, by contrast, has no fixed beginning or end and can therefore continue indefinitely. Since circular motion is continuous and uniform, it serves as the measure of other motions. Aristotle also notes that earlier philosophers treat change of place as the primary form of change. Since circular motion is the fundamental and eternal form of movement, it follows that the “first agent of change” (227) produces this motion.

Aristotle argues that the first agent of change “has no parts and no size” (227). His reasoning begins from the principle that nothing finite can produce change for an infinite time. If a finite magnitude possessed infinite power, it could produce an effect instantaneously, which is impossible. Likewise, an infinite magnitude cannot possess finite power. Therefore, Aristotle says, the eternal source of motion cannot be either finite or infinite in magnitude.

He also explains the motion of objects that continue moving after the initial mover has ceased contact, such as thrown bodies. In such cases, motion is transmitted through a sequence of intermediaries—like air or water—each briefly carrying the motion forward. This process is not truly continuous because the movers are successive rather than a single enduring cause.

True continuous motion requires a single mover acting continuously on a single magnitude. If the mover itself were moving, it would require another mover, leading to a regress. The chain must therefore terminate in something that moves other things while itself remaining unmoved. Aristotle concludes that the first mover must be indivisible and without magnitude. As it produces eternal motion and infinite duration of movement, it cannot possess finite size or any physical extension. This unmoved and immaterial source of motion is situated at the outer boundary of the universe. The first agent of movement, therefore, “causes eternal movement and does so for an infinite time” (231).