Sidgwick’s dualism of practical reason (Derek Parfit)
Derek Parfit summarizes Henry Sidgwick’s argument as follows:
(A) When we try to decide what we have most reason to do, we can rationally ask this question either from our own personal point of view or from an imagined impartial point of view.
(B) When we ask this question from our personal point of view, the answer is self-interested reasons are supreme.
(C) When we ask this question from an impartial point of view, the answer is that impartial reasons are supreme.
(D) To compare the strength of these two kinds of reason, we would need to have some third, neutral point of view.
(E) There is no such point of view.
(F) Impartial and self-interested reasons are wholly incomparable. When such reasons conflict, no reason of either kind could be stronger than any reason of the other kind.
Parfit, D. 2011. On What Matters, Volume 1. Oxford: Oxford University.