Thinking aloud. Most descriptions of chiral symmetry breaking nowadays present it as something happening in QCD. But it was defined well before of the quark theory, and then it was something related to isospin symmetry. It is a bit puzzling because it seems as if pion mass were originated from two different forces. On one hand, nuclear weak force breaks the symmetry between protons and neutrons and induces the mass of the pion. On other hand, nonzero quark masses break the symmetry between flavours and again the pion gets mass. In both scenarios the idea is that the pion should be massless because of the spontaneous symmetry breaking but it becomes massive because of the explicit symmetry breaking. Disturbing thing is that a pair of massive quarks up, down always break explicitly chiral symmetry but do not need to break, if they are equal mass, nuclear isospin symmetry.