Is a line really composed of points?

An Approach

t o

C A L C U L U S

Appendix 2

# IS A LINE REALLY COMPOSED OF POINTS?

THE WHOLE PROBLEM of a continuum of numbers began with the assumption, the concept, that a line—the x-axis—is composed of points, and that to every point there must correspond a number. But does calculus really require that? Let us begin by remembering that points and lines are mental objects. A line is the idea of length only. A point is the idea of position only. Points and lines exist as ideas. But is that sufficient? Should mathematics not be scientific, in the sense that we should be able to produce and witness what we speak of? Or is it enough for mathematics to deal with what can only be imagined? Should not a triangle we draw have more being for mathematics than an infinite line that we can only imagine?

Euclid postulated that a straight line will exist logically—when it has been drawn. We will have brought a representation of that idea into this world. As for a point, that is the name we give to the extremity of a line. Upon drawing a straight line, its extremities---those points---will logically exist. An instance of that idea will have been brought into this world.

In fact, we distinghish a finite straight line from an infinite line by saying that a finite line has two endpoints, while an infinite line has none.

With regard to the x-axis, we indicate points—positions—one at a time. We let a certain point have coördinate 0. We let the extremity of a distance from 0 have coördinate 1. We say, "Let x take values in the open interval between −4 and 5." Having thus indicated each point, that is all we need to mean when we say that that point logically exists.

(We may say there are an infinite number of points on a line, which is a brief way of saying that there is no limit to the number of points we could indicate.)

As for the x-axis being composed of points, that is completely at odds with the meaning of the word composed. For if something is composed, it is composed of parts. A class is composed of students; a cake is composed of ingredients. And each part exists independently. Each student exists independently of the class. The flour exists independently of the cake. If a straight line were composed of points, that would mean that the points exist independently of the line. Which is absurd. A point, in any event, is not a thing. "Point" is simply the name we give to a position.

Points exist potentially. They are like pitches on a guitar string. A pitch does not exist until it is sounded: a guitar string is not composed of pitches. And the x-axis is not composed of positions.

One could of course completely redefine the meaning of the word composed and the word point.

"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less."

"The question is," said Alice, "whether you can make words mean so many different things."

"The question is," said Humpty Dumpty, "which is to be master—that's all."

Alice in Wonderland

Say, however, that a line were composed of points, whatever that means. Now the most common and important application of calculus is to motion, where the independent variable is time t. Then if the abstract x-axis is composed of points, its application to time must also be composed of points. That is, time—the t-axis—will be composed of points, or, we would say, instants. Is that a valid presumption?

First, like any continuous quantity, time does not inherently consist of intervals; yet we can conveniently make the t-axis consist of any intervals,

any units of measure—hours, minutes, seconds—we please, however small.

Time will then be composed of those intervals, which will have common boundaries, to which we give the name "instants."

But time cannot be composed of instants, because they are not intervals. (To accept that an infinite number of points of zero time will add up to a positive time, not only calls for credulity more typical of the demands of religion; it approves division by 0.)

If time did consist solely of instants, then at any one instant a body is at rest. It cannot move to another instant, because no time elapses. In other words, there could be no motion. That is the arrow paradox of Zeno.  But because time continues and has no inherent components—it is not composed of instants—that paradox is not valid.

Since the t-axis then is not composed of points, then neither can the x-axis—the so-called real line—of which time is but an application.

Thus a continuum of numbers not only does not exist, it is not necessary. If a function is continuous, then it will be continuous at every value of x we name.

Appendix 1:  Are the real numbers really numbers?