  As a rule in General Relativity the spacetime metric fixes the Einstein
tensor and through the Field Equations (FE) the energy-momentum tensor. However
one cannot write the FE explicitly until a class of observers has been
considered. Every class of observers defines a decomposition of the
energy-momentum tensor in terms of the dynamical variables energy density
($\mu$), the isotropic pressure ($p$), the heat flux $q^a$ and the traceless
anisotropic pressure tensor $\pi_{ab}$. The solution of the FE requires
additional assumptions among the dynamical variables known with the generic
name equations of state. These imply that the properties of the matter for a
given class of observers depends not only on the energy-momentum tensor but on
extra a priori assumptions which are relevant to that particular class of
observers. This makes difficult the comparison of the Physics observed by
different classes of observers for the {\it same} spacetime metric. One way to
overcome this unsatisfactory situation is to define the extra condition
required among the dynamical variables by a geometric condition, which will be
based on the metric and not to the observers. Among the possible and multiple
conditions one could use the consideration of collineations. We examine this
possibility for the Friedmann-Lema\^{i}tre-Robertson-Walker models admitting
matter and Ricci collineations and determine the equations of state for the
comoving observers. We find linear and non-linear equations of state, which
lead to solutions satisfying the energy conditions, therefore describing
physically viable cosmological models.
