NOTE: Aristotle’s Physics takes its title from the Greek word  phusis, which translates more accurately as “the order of nature.” The first two books of the Physics are Aristotle’s general introduction to the study of nature. The remaining six books treat physics itself at a very theoretical, generalized level, culminating in a discussion of God, the First Cause.


Physics opens with an investigation into the principles of nature. At root, there must be a certain number of basic principles at work in nature, according to which all natural processes can be explained. All change or process involves something coming to be from out of its opposite. Something comes to be what it is by acquiring its distinctive form—for example, a baby becomes an adult, a seed becomes a mature plant, and so on. Since this the baby or the seed were working toward this form all along, the form itself (the idea or pattern of the mature specimen) must have existed before the baby or seed actually matured. Thus, the form must be one of the principles of nature. Another principle of nature must be the privation or absence of this form, the opposite out of which the form came into being. Besides form and privation, there must be a third principle, matter, which remains constant throughout the process of change. If nothing remains unchanged when something undergoes a change, then there would be no “thing” that we could say underwent the change. So there are three basic principles of nature: matter, form, and privation. For example, a person’s education involves the form of being educated, the privation of being ignorant, and the underlying matter of the person who makes the change from ignorance to education. This view of the principles of nature resolves many of the problems of earlier philosophers and suggests that matter is conserved: though its form may change, the underlying matter involved in changes remains constant.

Change takes place according to four different kinds of cause. These causes are closer to what we might call “explanations”: they explain in different ways why the change came to pass. The four causes are (1) material cause, which explains what something is made of; (2) formal cause, which explains the form or pattern to which a thing corresponds; (3) efficient cause, which is what we ordinarily mean by “cause,” the original source of the change; and (4) final cause, which is the intended purpose of the change. For example, in the making of a house, the material cause is the materials the house is made of, the formal cause is the architect’s plan, the efficient cause is the process of building it, and the final cause is to provide shelter and comfort. Natural objects, such as plants and animals, differ from artificial objects in that they have an internal source of change. All the causes of change in artificial objects are found outside the objects themselves, but natural objects can cause change from within.

Aristotle rejects the idea that chance constitutes a fifth cause, similar in nature to the other four. We normally talk about chance in reference to coincidences, where two separate events, which had their own causes, coincide in a way that is not explained by either set of causes. For instance, two people might both have their own reasons for being in a certain place at a certain time, but neither of these sets of reasons explains the coincidence of both people being there at the same time.

Final causes apply to nature as much as to art, so everything in nature serves a useful purpose. Aristotle argues against the views both of Democritus, who thinks that necessity in nature has no useful purpose, and of Empedocles, who holds an evolutionary view according to which only those combinations of living parts that are useful have managed to survive and reproduce themselves. If Democritus were right, there would be as many useless aspects of nature as there are useful, while Empedocles’ theory does not explain how random combinations of parts could come together in the first place.

Books III and IV examine some fundamental concepts of nature, starting with change, and then treating infinity, place, void, and time. Aristotle defines change as “the actuality of that which exists potentially, in so far as it is potentially this actuality.” That is, change rests in the potential of one thing to become another. In all cases, change comes to pass through contact between an agent and a patient, where the agent imparts its form to the patient and the change itself takes place in the patient.

Either affirming or denying the existence of infinity leads to certain contradictions and paradoxes, and Aristotle finds an ingenious solution by distinguishing between potential and actual infinities. He argues that there is no such thing as an actual infinity: infinity is not a substance in its own right, and there are neither infinitely large objects nor an infinite number of objects. However, there are potential infinities in the sense that, for example, an immortal could theoretically sit down and count up to an infinitely large number but that this is impossible in practice. Time, for example, is a potential infinity because it potentially extends forever, but no one who is counting time will ever count an infinite number of minutes or days.

Aristotle asserts that place has a being independent of the objects that occupy it and denies the existence of empty space, or void. Place must be independent of objects because otherwise it would make no sense to say that different objects can be in the same place at different times. Aristotle defines place as the limits of what contains an object and determines that the place of the earth is “at the center” and the place of the heavens as “at the periphery.”

Aristotle’s arguments against the void make a number of fundamental errors. For example, he assumes that heavier objects fall faster than lighter ones. From this assumption, he argues that the speed of a falling object is directly proportional to an object’s weight and inversely proportional to the density of the medium it travels through. Since the void is a medium of zero density, that would mean that an object would fall infinitely fast through a void, which is an absurdity, so Aristotle concludes that there cannot be such a thing as a void.

Aristotle closely identifies time with change. We register that time has passed only by registering that something has changed. In other words, time is a measure of change just as space is a measure of distance. Just as Aristotle denies the possibility of empty space, or void, Aristotle denies the possibility of empty time, as in time that passes without anything happening.


Aristotle’s conception of the natural world is based fundamentally on change. Rather than simply accept the fact that things change, Aristotle marvels at this fact and puzzles over how the world must be if change is possible. What change is and how it comes to pass sit at the heart of Aristotle’s scientific investigations. He investigates the fundamental principles of nature by asking what takes place in a process of change. He outlines four causes that explain change. He treats time as a measure of change. Later in the Physics, he expends a great deal of ingenuity on refuting paradoxes that suggest that change does not exist. This fascination with change allows Aristotle to look more deeply into the workings of nature than most of us would think to. By the end of book I, he claims to have discovered the three basic principles of nature without which change would be impossible. That is, by asking how it is that change might be possible, he develops a basic sense of how the universe must be arranged.

Aristotle’s investigation of the principles of matter leads him to draw the important distinction between form and matter. A classic example that illustrates this distinction is that of a bronze statue: the bronze is the matter, while the figure of the statue is the form. Neither matter nor form can exist independently. Even a lump of bronze would have some form, though the form would be less distinctive than that of a statue. Similarly, it would be impossible for a form to exist without some matter to take on that form. The statue need not be made of bronze to have its form, but it must be made of something. The form–matter distinction does a great deal of work for Aristotle, especially in the Physics and the Metaphysics, as it allows him to explain how something can both change and remain the same. If the bronze statue were melted down, for instance, the form would have changed but the matter would remain the same. If there were no unchanging matter, we would have no grounds for saying that the lump of bronze was in some way the same bronze as that which made up the statue.

Aristotle’s conception of change as being a process of something coming to be out of its opposite is troubling and does not sit well with his conception of the four causes. The idea gains strength from instances of change between binary opposites. For example, for something to become hot, it must have been colder before, so we can say that heat comes to be from out of its opposite, cold. However, there are many examples of change that do not mediate between binary opposites. In the summary just presented, we used the example of building a house when discussing the four causes. Aristotle might argue that the house comes to be from out of its opposite, which is “not a house,” but this is unconvincing. A house comes to be from out of a pile of bricks, wood, and mortar, and it seems far-fetched to argue that a pile of bricks, wood, and mortar are the opposite of a house. That same pile of bricks, wood, and mortar could be used to build many different kinds of structure, so Aristotle would have to say that the same pile of bricks, wood, and mortar is the opposite of an infinite number of possible buildings.

In his treatment of final causes, Aristotle boldly asserts that all of nature is teleological, meaning that it is organized toward a final end. In other words, he believes that all natural things have not only form and matter but also purpose. This belief in teleology deeply informs all of Aristotle’s work, from his scientific writings to his ethics. This belief also clashes sharply with modern conceptions of science, which explicitly does not try to identify purpose in the processes it observes. Aristotle’s conception of teleology in nature comes primarily from his impression of biological organisms, all of which are complex and highly efficient. Such organisms could not possibly come into being at random, he reasons, and so must all be designed with a particular purpose. It is interesting to note that, in arguing for this conclusion, Aristotle rejects an evolutionary conception of nature as advanced by Empedocles. Empedocles did not have the understanding of genetics or speciation that make modern evolutionary biology coherent, so Aristotle’s attack on Empedocles is valid. This fact may lead us to wonder, however, whether we might have developed modern evolutionary biology sooner than the nineteenth century had Aristotle not convinced his peers that Empedocles’ views were mistaken.