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

SYMBOLISM OF MATHEMATICS 65
Thus the first use of O was to make the
Arabic notation possible-no slight service.
We can imagine that when it had been introduced for this purpose, practical men, of the
sort who dislike fanciful ideas, deprecated the
silly habit of identifying it with a number
zero. But they were wrong, as such men
always are when they desert their proper
function of masticating food which others
have prepared. For the next service performed by the symbol O essentially depends
upon assigning to it the function of representing the number zero.
This second symbolic use is at first sight
so absurdly simple that it is difficult to make
a beginner realize its importance. Let us
start with a simple example. In Chapter II
we mentioned the correlation between two
variable numbers x and y represented by the
equation x+y=1. This can be represented
in an indefinite number of ways; for example,
x = 1-y, y = 1-x, 2x+3y-1 l=x+2y, andso
on. But the important way of stating it is
x+y-1 =0.
Similarly the important way of writing the
equation x = 1 is x -1 = 0, and of representing
the equation 3x-2=2x2 is 2x2-3x+2=0.
The point is that all the symbols which represent variables, e.g. x and y, and the symbols