What is a state vector?

A state vector is not a property of a physical system (nor of an ensemble of systems). It does not evolve continuously between measurements, nor suddenly ``collapse'' into a new state vector whenever a measurement is performed. Rather, a state vector represents a procedure for preparing or testing one or more physical systems. No ``quantum paradoxes'' ever appear in this interpretation. The formulation of dynamical laws may involve path integrals and/or S-matrix theory.