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

44 INTRODUCTION TO MATHEMATICS
form motion in a straight line, except so far
as it is compelled by impressed force to
change that state. This law is more than a
dry formula: it is also a poean of triumph
over defeated heretics. The point at issue
can be understood by deleting from the
law the phrase "or of uniform motion in a
straight line." We there obtain what might
be taken as the Aristotelian opposition
formula: "Every body continues in its state
of rest except so far as it is compelled by
impressed force to change that state."
In this last false formula it is asserted that,
apart from force, a body continues in a state
of rest; and accordingly that, if a body is
moving, a force is required to sustain the
motion; so that when the force ceases, the
motion ceases. The true Newtonian law
takes diametrically the opposite point of
view. The state of a body unacted on by
force is that of uniform motion in a straight
line, and no external force or influence is to
be looked for as the cause, or, if you like to
put it so, as the invariable accompaniment
of this uniform rectilinear motion. Rest is
merely a particular case of such motion,
merely when the velocity is and remains zero.
Thus, when a body is moving, we do not seek
for any external influence except to explain
changes in the rate of the velocity or changes
in its direction. So long as the body is moving