The objective of the paper is to provide a critical exposition of Henry Sidgwick's theory of "proof" in ethics, by means of a restatement and a critique of relevant sections of Book IV of The Methods of Ethics and an article in the 1879 volume of Mind. It is concluded that Sidgwick's thought contains two fundamental unresolved tensions. One of these relates to whether "proof" is to be treated as a normative or an empirical matter. On the one hand, Sidgwick (...) clearly wants to offer a ground for ethics whose epistemic force would be universal; on the other, he accepts Mill's "considerations determining the mind to accept" as a definition. The second unresolved tension relates to the question whether abstract transcendent axioms or the familiar rules of common sense morality constitute the ultimate court of appeal in ethical decisionmaking. (shrink)