Therefore, we define the operator S i + and S i − − . In the case of the non-relativistic particle, if S + is applied to a fermion twice, the resulting eigenvalue is 0. Similarly, the eigenvalue is 0 when S − − is applied to a fermion twice. This relation satisfies the Pauli Exclusion Principle. However, bosons are symmetric particles, which do not obey the Pauli Exclusion Principle.