In theoretical studies of the behaviour of materials it is necessary to introduce postulates about the way in which materials respond to factors such as local deformations and local temperature gradients. These postulates naturally depend on the type of material under consideration and in continuum theories they are expressed mathematically in the form of certain relations between tensors called constitutive equations. A simple example of a constitutive equation is that relating the stress components σij and the pressure p in an inviscid fluid, namely