Table of Contents
Book I
At the end of Book I of Aristotle’s Physics, he claims that he has sufficiently argued that there are principles, that he has shown what those principles are, and that he has determined how many of them there are. Why should we care about principles? Because principles are the doors to scientific knowledge. Aristotle draws on his predecessors, who all agree that contraries are principles. Examples of contraries include excess and defect, above and below, before and behind, angular and angle-less, straight and round, and so forth. If contraries come in pairs and contraries are principles, then in the abstract, there are 2 principles. As we observe things in our world, we notice that their properties tend to change between the contraries. Things change from hot to cold, wet to dry, e.t.c and vice versa. The things in which the contraries display their properties and which persist through the change is called the substratum, which is the third principle.
Book II
Book II gives an account of the primary causes. What is a cause? It is that from which a thing comes to be and that from which it persists. In a single paragraph Aristotle summarizes all of them:
“All the causes now mentioned fall into four familiar divisions. The letters are the causes of syllables, the material of artificial products, fire, &c., of bodies, the parts of the whole, and the premises of the conclusion, in the sense of ‘that from which’. Of these pairs the one set are causes in the sense of substratum, e.g. the parts, the other set in the sense of essence-the whole and the combination and the form. But the seed and the doctor and the adviser, and generally the maker, are all sources whence the change or stationariness originates, while the others are causes in the sense of the end or the good of the rest; for ‘that for the sake of which’ means what is best and the end of the things that lead up to it. (Whether we say the ‘good itself’ or the ‘apparent good’ makes no difference.)”
The first set of causes; the ones that define the letters of syllables, the material of artificial products, the parts of the whole, etc.; are those that Aristotle describes as “that from which.” In general, these correspond to form and matter, where matter, for example, the substratum, which we discovered in Book I to be a principle, is also a cause. The third cause comes from “all sources whence change and stationariness originate”; this is the efficient cause. The “that for the sake of which” is called the final cause.
Book III
In Book III, we defined nature as a principle of motion. Since nature is a principle of motion, we cannot understand nature before we understand motion. Since motion is continuous, and the infinite presents itself in the continuous, we must understand the infinite first and foremost. We must understand the way in which the infinite exists, the way in which it does not exist, and what it is. Since motion is concerned with bodies, we ask: does an infinite body exist? A proof that an infinite body does not exist goes as follows:
- The definition of a body is “bounded by a surface.”
- The infinite is unbounded.
- Therefore, an infinite body cannot exist because it would be unbounded.
Now, given that an infinite body does not exist, that does not mean that the infinite does not exist in other ways. The infinite must exist because, if it did not, time would not be infinite.
Book IV
Book IV addresses place, void, and time.
Place cannot be a body because, if it were, there would be two bodies in the same place, which is impossible. Place is, in a sense, a home for the body, and bodies have their natural places: the place for fire is up, and the place for earth is down. Thus, these bodies tend to move up or down to their appropriate places. What is the cause of place? It cannot be matter or form, since form and matter are not separate from things, whereas place can be separated from things. For example, an empty cup is filled with air, but when water is poured into it, the air exits the cup, and the cup is then filled with water. The place that contained air now contains water, but it is still the same place, independent of what body occupies it.
While place exists, the void, on the other hand, does not. What is the void? The void is a place without a body. It is very hard to imagine such a thing, because when air makes room for water, the air must move to some other place. The existence of a void would mean a world in which things do not move from place to place. This is because, unlike place, the void yields to any body. If so, then a body would move into any empty place, and something like fire, which would normally move up, would be equally justified in moving in any direction, including down.
What about time? Does it exist? What is its nature? Previous thinkers thought that time is motion. They saw things around them moving and took that to be what time is. But motion is not time. Motion exists only in moving things, whereas time is everywhere, in moving things and in things at rest. Even though time is not motion, it is related to motion: if the state of our mind does not change (that is, if it is not in motion), we would not be aware of the passage of time. How is this relation perceived? Things in motion are described in terms of “before” and “after”: before a body moved, it was located at X; after the body moved, it was located at Y. Time also has this distinction of “before” and “after” because it is related to movement. We are only qualified to claim that time has elapsed when we have sensed the “before” and “after” of a thing in motion. In “before” and “after”, there are two “nows”. “Now” the object is at X; then, after waiting for a period of time, we identify a second now: “now the object is at Y.” The “nows” are what bound time. If there is only one “now”, there is no time, because nothing has moved. Only when there is more than one “now” do we say there is time. So time is the number of motion in respect to “before” and “after”.
Discuss on Github