
The habit of lining sheep up in a row so that they can easily be counted leads to the concept of
a number line, that is, a line with the numbers marked off at regular intervals, as in Figure 2.2. This
line can, in principle, go on for as long as we wish, but to avoid boredom we have stopped at five
sheep and put on an arrowhead to let you know that the line can continue. Clear thinkers can visu
-
alize it going off to infinity, but historical purveyors of dead sheep probably gave this concept little
thought, outside of their dreams and fevered imaginings.
At some point in history it was probably realized that you can sometimes, if you are a particu
-
larly fast talker, sell a sheep that you don’t actually own, thus, simultaneously inventing the
important concepts of debt and negative numbers. Having sold this putative sheep, you would in
fact own “negative one” sheep. This would lead to the discovery of integers, which consist of the
natural numbers and their negative counterparts. The corresponding number line for integers is
shown in Figure 2.3.
The concept of poverty probably predated that of debt, leading to a growing number of people who
could afford to purchase only half a dead sheep, or perhaps only a quarter. This lead to a burgeon-
ing use of fractional numbers, consisting of one integer divided by another, such as 2/3 or 111/27.
Mathematicians called these rational numbers, and they fit in the number line in the obvious
places between the integers. At some point, people became lazy and invented decimal notation,
like writing “3.1415” instead of the longer and more tedious 31415/10000.
After a while, it was noticed that some numbers that appear to turn up in everyday life are not
expressible as rational numbers. The classic example is the ratio of the circumference of a circle to
its diameter, usually denoted as p (pronounced “pi”). These are the so-called real numbers, which
include rational numbers and numbers such as p that would, if expressed in decimal form, require
an infinite number of decimal places. The mathematics of real numbers is regarded by many to be
the most important area of mathematics, and since it is the basis for most forms of engineering, it
can be credited with creating much of modern civilization. The cool thing about real numbers is
that while rational numbers are countable (that is, placed into one-to-one correspondence with the
natural numbers), real numbers are uncountable. The study of natural numbers and integers is
called discrete mathematics, and the study of real numbers is called continuous mathematics.
The truth is, however, that real numbers are nothing more than a polite fiction. They are a rela
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tively harmless delusion, as any reputable physicist will tell you. The universe seems to be not
only discrete, but also finite. If there are a finite amount of discrete things in the universe, as cur
-
rently appears to be the case, then it follows that we can only count to a certain fixed number.
Thereafter, we run out of things to count — not only do we run out of dead sheep, but toasters,
mechanics, and telephone sanitizers also. It follows that we can describe the universe using only
Chapter 2: The Cartesian Coordinate System
7
Figure 2.3: A
number line for
integers (note the
ghost sheep for
negative
numbers)