To have Lorentz invariance, the time derivatives and spatial derivatives must be treated equally (i.e. of the same order). Therefore, it is convenient to choose the Lorenz gauge condition, which makes the left hand side zero and gives the result ◻ ◻ A = [ 1 c 2 ∂ ∂ 2 ∂ ∂ t 2 − − ∇ ∇ 2 ] A = μ μ 0 J .