Equation 3.46 shows that the internal energy of a closed system changes in a simple
way when either S or V is changed (dU ∝ dS and dU ∝ dV). These simple proportionalities
suggest that U is best regarded as a function of S and V. We could regard U
as a function of other variables, such as S and p or T and V, because they are all interrelated;
but the simplicity of the fundamental equation suggests that U(S,V) is the
best choice.