This relation also holds for classical mechanics, the classical limit of the above, given the correspondence between Poisson brackets and commutators, [ A , H ] ↔ ↔ i ℏ ℏ { A , H } In classical mechanics, for an A with no explicit time dependence, { A , H } = d d t A , so, again, the expression for A (t) is the Taylor expansion around t = 0.