In a PVT system, knowing the function U(S,V) (or S(U,V) ) allows us to know all the information about the system, so it's called the fundamental equation of thermodynamics (and it makes sense to me because it's derived from the 1st and 2nd law). But if we know the function P(V,T) for example we also know all the information (all the state variables) about the system, so why isn't that the fundamental equation? I guess what I really want to know is what is "knowing all the information about the system"... The difference I see is that knowing U(S,V) allows us to know what will be the new state variables if there is work or heat transferred to the system, while only knowing P(V,T) as far as I know doesn't allows us to see that. So I guess the difference between the 2 is that knowing U(S,V) allows us to know how the system will react to thermodynamic processes, while P(V,T) doesn't?