  In this paper, it is pointed out for the first time that the linear effective
potential between quarks is intrinsically relating to the bag model while
concerning the asymptotically-free nature of colours. Based on the relationship
we employ the symmetry method to analyze the quark-anti-quark system. For the
tight bound system, one of the quark and anti-quark becomes the bag itself. By
imposing the Poincare invariance on the quark-anti-quark bound-state, and
translating the chiral transformation to its spatial manifestation, we can
infer why the chiral symmetry breaking happens. Applying this knowledge to deep
inelastic scattering we reach the conclusion that the measured proton spin in
scattering experiments should be uncertain quantity.
