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

DYNAMICS 49
anyone in particular, or to any special modes
of perception. The result is that the chair
becomes in thought a set of molecules in
space, or a group of electrons, a portion of
the ether in motion, or however the current
scientific ideas describe it. But the point is
that science reduces the chair to things
moving in space and influencing each other's
motions. Then the various elements or
factors which enter into a set of circumstances, as thus conceived, are merely the
things, like lengths of lines, sizes of angles,
areas, and volumes, by which the positions
of bodies in space can be settled. Of course,
in addition to these geometrical elements the
fact of motion and change necessitates the
introduction of the rates of changes of such
elements, that is to say, velocities, angular
velocities, accelerations, and suchlike things.
Accordingly, mathematical physics deals with
correlations between variable numbers which
are supposed to represent the correlations
which exist in nature between the measures
of these geometrical elements and of their
rates of change. But always the mathematical laws deal with variables, and it is
only in the occasional testing of the laws by
reference to experiments, or in the use of
the laws for special predictions, that definite
numbers are substituted.
The interesting point about the world as