  We describe both quantum particles and classical particles in terms of a
classical statistical ensemble, characterized by a probability distribution in
phase space. By use of a wave function in phase space both can be treated in
the same quantum formalism. The different dynamics of quantum and classical
particles resides then only from different evolution equations for the
probability distribution. Quantum particles are characterized by a specific
choice of observables and time evolution of the probability density. All
relations for a quantum particle in a potential, including interference and
tunneling, can be described in terms of the classical probability distribution.
We formulate the concept of zwitters - particles for which the time evolution
interpolates between quantum and classical particles. Experiments can test a
small parameter which quantifies possible deviations from quantum mechanics.
