What does it mean to say that the fundamental relation of thermodynamics is homogeneous first-order? I struggle with the abstraction of mathematical definitions and, I'm really seeking more to understand the relation of the physical variables of thermodynamics. Why are all of the variables interdependent by a linear relationship? Or, is it incorrect to say such a thing? Is it because, you look at the change in every variable as a function of the changes of every other variable? Please know, the time and assistance of all those who respond is deeply appreciated. Sincere thanks. M.