I used to wrestle with that until I realized that the issue is a bit like a diagnosis: you present with, say, a fever and a few other symptoms, and you are given a diagnosis of X. That's all at the surface; for me first principles became a recursive dive into ever-more-deep subjects.
Of course, you could recurse all the way down to the mean free paths of molecules, but, in reality, first principles starts somewhere way above that, maybe at, say, molecular binding (proteins and so forth).
Of course, you could recurse all the way down to the mean free paths of molecules, but, in reality, first principles starts somewhere way above that, maybe at, say, molecular binding (proteins and so forth).