An introduction to mathematics, by A. N. Whitehead.

92 INTRODUCTION TO MATHEMATICS
when found, was a much more elaborate affair
than that of the negative numbers and the
reader's attention must be asked for some
careful preliminary explanation. We have
already come across the representation of a
point by two numbers. By the aid of the
P' /_ p
y y." y
y' x' O x M -,
N' y'
P"' plit
Fig. 8.
positive and negative numbers we can now
represent the position of any point in a plane
by a pair of such numbers. Thus we take
the pair of straight lines XOX' and YOY', at
right angles, as the "axes" from which we
start all our measurements. Lengths measured along OX and OY are positive, and
measured backwards along OX' and OY' are
negative. Suppose that a pair of numbers,
written in order, e.g. (+3, +1), so that there