  This essay is an attempted to address, from a modern perspective, the motion
of a particle. Quantum mechanically, motion consists of a series of
localizations due to repeated interactions that, taken close to the limit of
the continuum, yields a world-line. If a force acts on the particle, its
probability distribution is accordingly modified. This must also be true for
macroscopic objects, although now the description is far more complicated by
the structure of matter and associated surface physics.
