What is Sidgwick's dualism of practical reason?

Sidgwick's Dualism of Practical Reason has attracted the attention of many interpreters, and the Dualism itself seems to be an historically important version of the view, recently defended by Thomas Nagel, Susan Wolf, and others, that there exists a fundamental fragmentation of value that the cosmos of duty is reduced to chaos, in Sidgwick's words. In this paper, I consider and reject the leading interpretations of Sidgwick's Dualism, and propose an alternative reading. I conclude by offering what I hope is a plausible but Sidgwickian way out of the Dualism.