Computation is widely held to be substrate-independent: a program is fixed by an automaton and runs on any hardware that implements it. We show that an implemen- tation map cannot be at once instantaneous (its verdict a function of the current state), neutral (specified from the automaton alone, blind to substrate timescales), and faithful (its readout tracks the automaton’s transitions) — at most two of the three. The proof uses one structural fact, a two-directional Myhill–Nerode lemma, from w