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

SYMBOLISM OF MATHEMATICS 69
number. This is done by turning the numbers
which occur in the definite correlation 3x +2y
-5 =0 into letters. We obtain ax +by -c =0.
Here a, b, c stand for variable numbers just
as do x and y: but there is a difference in the
use of the two sets of variables. We study
the general properties of the relationship between x and y while a, b, and c have unchanged values. We do not determine what
the values of a, b, and c are; but whatever
they are, they remain fixed while we study
the relation between the variables x and y
for the whole group of possible values of x
and y. But when we have obtained the
properties of this correlation, we note that,
because a, b, and c have not in fact been determined, we have proved properties which must
belong to any such relation. Thus, by now
varying a, b, and c, we arrive at the idea that
ax+by-c = represents a variable linear
correlation between x and y. In comparison
with x and y, the three variables a, b, and c
are called constants. Variables used in this
way are sometimes also called parameters.
Now, mathematicians habitually save the
trouble of explaining which of their variables
are to be treated as "constants," and which
as variables, considered as correlated in their
equations, by using letters at the end of the
alphabet for the "variable" variables, and
letters at the beginning of the alphabet for