Given vectors $\boldsymbol { a } , \boldsymbol { b }$, then ``$( \boldsymbol { a } + \boldsymbol { b } ) ( \boldsymbol { a } - \boldsymbol { b } ) = 0$'' is ``$\boldsymbol { a } = \boldsymbol { b }$ or $\boldsymbol { a } = - \boldsymbol { b }$'' a \_\_\_\_ condition.