He noticed that if this could be done, then H, considered as a matrix function of X and P, will have zero time derivative. d H d t = P ∗ ∗ d P d t + ( X + 3 ε ε X 2 ) ∗ ∗ d X d t = 0 , where A ∗ B is the anticommutator, A ∗ ∗ B = 1 2 ( A B + B A ) .