The 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.
The 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.