The materials considered in this paper exhibit the following phenomena: (1) a time‐dependent stress approaching a steady value with increasing time in a simple‐tension relaxation test, and (2) a time‐dependent strain rate approaching a steady value with increasing time in a simple‐tension creep test. They are also substantially isotropic and incompressible. Such materials are very common, but, as is shown in this paper, continuum models which will represent them are not. Even very general models such as the Rivilin‐Ericksen and hygrosteric models are shown to be inadequate for this purpose. Models of function type, which do admit the phenomena described above, are considered here within the domain of finite deformations and nonlinear material response. The level of generality of the models is such that the variables appearing in the constitutive equation may be chosen but the form of the function relating them may not. This level forms the groundwork for specialized classification, and leaves open the possibility that a theoretical program of experimentation may be constructed which allows a given material to numerically identify the form of its function within the framework of the general model. As a first step in the search for a continuum model for these materials, the following question arises. Within the level of generality described, what is the simplest model of function type which will represent the material behavior discussed? Re‐expressed: what is the least number of variables which must appear in the constitutive equation, and what are these variables? These questions are answered in this paper.