could someone explain the following sentence to me? "First-order perturbation correction is often precluded because of a symmetry principle operating for the state under consideration." "In second-order, on the other hand, the perturbation has to connect the given state with some other state and back, and therefore, its contribution is no longer inhibited by symmetry." I can comprehend the mathematical skills of the perturbation theory. However, depending on the circumstance I do not understand the physical basis how first-order perturbation can become zero due to symmetry. 2. peopel say the cause of degeneracy is related with symmetry. then how is the symmetry related with degeneracy ? Since I am a Korean, please explain your answer in qualitative simple phrases.