This paper examines the undervalued role of Herbert Spencer in Sidgwick's thinking. Sidgwick recognized Spencer's utilitarianism, but criticized him on the ground that he tried to deduce utilitarianism from evolutionary theory. In analysing these criticisms, this paper concludes that Spencer's deductive methodology was in fact closer to Sidgwick's empiricist position than Sidgwick realized. The real source of Sidgwick's unhappiness withSpencer lies with the substance of Spencer's utilitarianism, namely its espousal of indefeasible moral rights.