  We formulate the Schr\"odinger equation as the equation of motion of a small
external influence which serves as the initial boundary condition of a physical
system in classical laboratory space. The Hilbert space of possible external
influences for a given physical system is then equivalent to the Hilbert space
of a quantum system without spin. We discuss the double-slit experiment in the
context of this approach and show that wave-particle dualism reduces to the
choice of a basis in the geometrical construction of the Hilbert space of small
external influences.
