Chapter 4 discussed simple models for solids – in which the stress is determined by the strain – and Chapter 6 discussed fluids – in which the stress is determined by the strain rate. Both these sets of models for solids and fluids are time-independent in the sense that the stress at any instant of time is determined, respectively, by the values of the strain and the strain rate at that instant of time; the response of the material to any stimulus (stress or deformation) is instantaneous. In essence, the past histories of the strain and strain rate have no effect on the current stress.