 Notwithstanding the undervalued similarities between their respective versions of evolutionary utilitarianism, Spencer and Sidgwick nevertheless parted company in two fundamental respects. First, whereas for Spencer, “rational” utilitarianism refines “empirical” utilitarianism by converging on indefeasible moral rights, for Sidgwick, systematization never ceases. Rather, systematizing common sense continues indefinitely in order to keep pace with the vicissitudes of our social circumstances. The best utilitarian strategy requires flexibility and not the cramping rigidity of unyielding rights. In effect, Spencer’s utilitarianism was too dogmatically liberal for Sidgwick’s more tempered political tastes.