Desires, values, reasons, and the dualism of practical reason

In On What Matters Derek Parfit argues that facts about reasons for action are grounded in facts about values and against the view that they are grounded in facts about the desires that subjects would have after fully informed and rational deliberation. I describe and evaluate Parfit's arguments for this value-based conception of reasons for action and find them wanting. I also assess his response to Sidgwick's suggestion that there is a Dualism of Practical Reason. Parfit seems not to notice that his preferred value-based conception of reasons for action augurs strongly in favour of a view like Sidgwick's.