In some papers, the following paradox was proposed. The description of the internal energy of a simple gas is based historically on free expansion experiments and univariable equations of state. According to these equations, the internal energy of simple monatomic, diatomic, and polyatomic gases can be expressed only as a function of temperature. The authors declared that it was a wrong approach because a thermodynamic state cannot depend on one variable but must depend on two. They claim that multivariable equations of state derived from composite functions must be used instead which say that the internal energy of these gases can in each situation also be expressed only as a function of volume and pressure. In the present paper, an attempt to resolve this paradox ismade. This paper could be of interest to a broad range of chemists and chemical engineers who use thermodynamics of gases in their work.