Hopefully this is the right place to post this question. This is a very fundamental question on the applicability and limitations of classical equilibrium thermodynamics (CET). I've been learning non-equilibrium thermodynamics (NET), and a few sources mentioned that NET is needed because CET is only limited to reversible processes. In the press release for the 1977 Nobel Prize in Chemistry (went to Prigogine for his work on NET), it's mentioned that CET "cannot be used for the study of irreversible processes but only for reversible processes and transitions between different states of equilibrium." This is something I've also seen mentioned in various sets of lecture notes that I found online. So this leads me to my question. Is it a meaningless question to ask whether a process is reversible or irreversible in the context of CET? Because it seems like one of the assumptions of applying CET is that it only applies to reversible processes, so whenever you apply CET, everything must be a reversible process, even when you have friction and whatnot (whether or not this gives accurate results is a separate issue).