The intrinsic parity's phase is conserved for strong and electromagnetic interactions (the product of the intrinsic parities is the same before and after the reaction), but not for weak interactions. As [ P , H ] = 0 the Hamiltonian is invariant under a parity transformation. The intrinsic parity of a system is the product of the intrinsic parities of the particles, for instance for noninteracting particles we have P ( | 1 ⟩ ⟩ | 2 ⟩ ⟩ ) = ( P | 1 ⟩ ⟩ ) ( P | 2 ⟩ ⟩ ) . Since the parity commutes with the Hamiltonian and d P d t = 0 its eigenvalue does not change with time, therefore the intrinsic parities phase is a conserved quantity.